To determine the optimal extent to which Acea S.P.A. should utilize hybrid bonds relative to the S&P Global Ratings cap, we must analyze the company's financial position, the cost of capital environment, and the strategic benefits of hybrid debt within the constraints of the rating agency's methodology.

**1. Financial Health and Capital Structure Analysis (2022 Data):**
*   **Total Equity:** €2,755,243,000
*   **Total Liabilities:** Total Assets (€11,338,533,000) - Total Equity (€2,755,243,000) = €8,583,290,000.
*   **Financial Debt:**
    *   Other Noncurrent Financial Liabilities: €4,722,263,000
    *   Other Current Financial Liabilities: €619,418,000
    *   Total Financial Debt ≈ €5,341,681,000.
*   **Leverage Ratio:** Debt / (Debt + Equity) ≈ 5.34 / (5.34 + 2.76) ≈ 66%. This indicates a moderately leveraged utility company, typical for the sector which is capital intensive.
*   **Profitability:** The company is profitable with a Net Profit of €311,160,000 and positive Operating Cash Flow of €726,703,000. This supports the ability to service additional debt-like instruments, provided the coupon deferral options (typical of hybrids) are not exercised unnecessarily.

**2. Market Environment and Cost of Capital:**
*   **Interest Rates:** The swap curves show a dramatic increase in rates from 2021 to 2022 (e.g., 10Y swap average went from 0.053% to 1.927%). This makes traditional fixed-rate debt more expensive.
*   **Credit Spreads:** The iBoxx EUR Non-Financial IG spread increased from 1.298% in 2021 to 2.295% in 2022.
*   **Hybrid Advantage:** Hybrid bonds typically offer a higher coupon than senior debt but are treated as equity for rating purposes (up to the cap). In a rising rate environment, issuing hybrids can be attractive if the equity credit allows the company to maintain its investment-grade rating without issuing expensive senior debt or dilutive equity. The "equity credit" reduces the leverage ratio calculated by rating agencies, protecting the credit rating.

**3. S&P Global Ratings Methodology and Cap:**
*   S&P Global Ratings typically allows an equity credit of up to 100% of the hybrid instrument's value, but this credit is capped at a certain percentage of the company's **Total Adjusted Capital** (Debt + Equity).
*   The prompt specifies a cap where equity credit is limited to a maximum of **15% of total adjusted capital**.
*   **Strategic Optimization:** Companies generally aim to maximize the benefit of hybrid instruments to optimize their Weighted Average Cost of Capital (WACC) and leverage ratios. By utilizing hybrids up to the maximum allowed cap, a company can:
    1.  Strengthen its balance sheet from a rating agency perspective (lowering reported leverage).
    2.  Avoid the higher cost and dilution of issuing pure equity.
    3.  Maintain financial flexibility compared to senior debt (due to coupon deferral features).

**4. Determining the Extent of Utilization:**
*   The question asks to what extent the company *should* utilize hybrid bonds *relative to the cap*.
*   Given that Acea is a stable, profitable utility with significant infrastructure investment needs (CapEx was over €1 billion in 2022), maintaining a strong credit rating is crucial for low-cost funding.
*   There is no indication of financial distress that would preclude the use of hybrids. In fact, the high leverage (66%) suggests that boosting the equity base via hybrid instruments (which count partially as equity) is beneficial to de-lever the balance sheet in the eyes of rating agencies.
*   Therefore, the optimal strategy is to utilize the hybrid instrument capacity **fully** up to the limit permitted by the rating agency to maximize the equity credit benefit. Using less than the cap (e.g., 0%, 25%, 50%, 75%) would leave "cheap" equity credit on the table, potentially resulting in a lower credit rating or higher cost of senior debt than necessary.
*   The "cap" defines the maximum efficient amount. Rational financial management dictates utilizing the available efficient capital structure tools to their maximum beneficial extent. Thus, the company should utilize hybrids up to 100% of the allowed cap.

<answer>100%</answer>