To determine the appropriate extent to which Bouygues should utilize hybrid bonds, we must analyze its financial position, capital structure needs, and the specific guidance provided for hybrid issuance levels.

**1. Financial Position and Leverage Analysis (FY 2022)**
*   **Equity:** Total Equity is €13,932 million.
*   **Debt:** Long-term borrowings are €11,586 million. Current borrowings are €1,361 million. Net Debt increased significantly from €941 million in 2021 to €7,440 million in 2022.
*   **Net Debt Increase Driver:** The primary driver for the increase in Net Debt and the change in the balance sheet structure is the "Cash Flows Used In Obtaining Control Of Subsidiaries" of €6,269 million. This indicates a transformational M&A activity (likely the acquisition of a significant stake in a subsidiary, such as the increase in Bouygues Telecom or similar consolidation events common for the group).
*   **Profitability:** Profit attributable to owners is €973 million. Interest expense is €231 million. The company is profitable and generates positive operating cash flow (€2,978 million from operating activities).

**2. Assessment of Capital Structure Needs**
*   **M&A Impact:** The substantial cash outflow for acquisitions (€6.27 billion) has materially increased the company's leverage profile. While the company remains investment grade, such large-scale acquisitions typically strain traditional debt metrics.
*   **Refinancing and Optimization:** The jump in Net Debt suggests a need to optimize the capital structure to maintain credit ratings. Hybrid bonds are an effective tool for this because they are treated as equity for rating agency purposes (up to a limit), thereby lowering reported leverage ratios (Debt/EBITDA, Net Debt/Equity).
*   **Current Hybrid Status:** The prompt asks to consider the "current starting point of outstanding hybrid bonds." While the exact breakdown of existing hybrids isn't explicitly itemized as a separate line item in the provided summary, Bouygues historically utilizes hybrids. However, the key factor here is the *incremental* need due to the 2022 M&A activity.

**3. Evaluating the Options against Guidelines**

*   **0%:** Incorrect. The company has high refinancing/acquisition needs (Net Debt rose by ~€6.5 billion). Ignoring hybrid issuance would leave leverage higher than necessary, potentially pressuring the rating.
*   **3.75%:** This level is for "mild leverage optimization." Given the scale of the acquisition (€6.27 billion) relative to the equity base (€13.9 billion), the leverage impact is more than "mild." The Net Debt to Equity ratio has deteriorated significantly.
*   **7.5%:** This level corresponds to "Moderate refinancing or acquisition needs" and where "Hybrid issuance meaningfully improves adjusted leverage metrics." The acquisition was significant but not necessarily "transformational" in a way that threatens the entire business model's survival without hybrid support (unlike a 15% scenario). The company has strong operating cash flows (€2.98 billion) to service debt. Issuing hybrids up to 7.5% of adjusted capital would help absorb the leverage shock from the acquisition and provide rating headroom.
*   **11.25% - 15%:** These levels are reserved for "High capex intensity OR large acquisition pipeline" with "Significant leverage pressure" or "Material downgrade risk." While the acquisition was large, Bouygues is a diversified conglomerate (Construction, Media, Telecom) with stable cash flows. The leverage increase, while notable, is manageable within the Investment Grade spectrum without needing to max out hybrid capacity. The cost of hybrids is generally higher than senior debt; issuing too much (11.25-15%) would unnecessarily increase the weighted average cost of capital (WACC) when a moderate amount (7.5%) suffices to stabilize metrics.

**4. Calculation Check (Approximate)**
*   Total Adjusted Capital ≈ Equity + Debt.
*   Equity ≈ €13.9 billion.
*   Gross Debt ≈ €12.9 billion (Long term + Current).
*   Total Capital ≈ €26.8 billion.
*   7.5% of €26.8 billion ≈ €2.0 billion.
*   The prompt allows up to €3 billion per year.
*   Issuing ~€2 billion in hybrids would effectively treat that amount as equity for rating purposes, significantly de-levering the balance sheet post-acquisition. This aligns with the "meaningfully improves adjusted leverage metrics" criterion for the 7.5% tier.

**Conclusion:**
The company has undergone significant M&A activity leading to higher net debt. To optimize the capital structure and protect its credit rating without incurring excessive interest costs associated with higher hybrid volumes, a moderate issuance is appropriate. This fits the profile for **7.5%**, which addresses moderate acquisition needs and provides meaningful leverage improvement.

<answer>7.5%</answer>