**Reasoning**

1. **Company profile** – Iberdrola S.A. is a large regulated utility with a strong market position in Spain and internationally. It generates stable cash flows (≈ €10.4 bn operating cash flow in 2022) but also has high capital‑intensive growth (capex ≈ €6.3 bn in 2022) and a sizable debt maturity profile (≈ €10 bn short‑term financial debt).

2. **Total adjusted capital**  
   - Equity (including non‑controlling interests) = €58.1 bn.  
   - Adjusted financial debt (bank & bonds ≈ €46.6 bn, lease liabilities ≈ €2.4 bn, other financial liabilities ≈ €6.6 bn) ≈ €55.6 bn.  
   - **Total adjusted capital** ≈ €113.7 bn.

3. **Current hybrid instruments**  
   - Perpetual subordinated bonds issued in 2021: €2.74 bn.  
   - Other hybrid‑type capital instruments (non‑current + current) ≈ €0.66 bn.  
   - **Total hybrid principal** ≈ €3.4 bn (≈ 3 % of total adjusted capital).  
   - Equity credit (50 % of hybrid) ≈ €1.7 bn, i.e. **≈ 1.5 % of total adjusted capital**, well below the 15 % cap.

4. **Funding environment and credit considerations**  
   - The company faces moderate‑to‑high refinancing needs (short‑term maturities ≈ €10 bn) and a large capex pipeline for renewable expansion.  
   - Leverage (debt/EBITDA ≈ 3.5×) and interest coverage (EBIT/interest ≈ 2.6×) leave limited rating headroom; the rating is likely in the ‘BBB+’ band.  
   - Issuing hybrid capital would improve adjusted leverage, provide flexibility for the capex programme, and help preserve the investment‑grade rating without a material increase in cost of debt.

5. **Guideline check**  
   - **0 %** – not appropriate: the company has substantial refinancing and capex needs, and a modest amount of hybrid already exists.  
   - **3.75 %** – would only marginally raise the hybrid level; insufficient to meaningfully affect leverage or rating headroom.  
   - **7.5 %** – aligns with moderate refinancing/acquisition needs, moderately constrained rating headroom, and the ability of hybrid issuance to improve adjusted leverage metrics at a manageable cost increase.  
   - **11.25 % / 15 %** – would require a larger issuance programme (beyond the 3 bn €/year limit for the next 18 months) and would be excessive given the current leverage profile; the company does not face a “very high” refinancing or transformational M&A scenario that would demand such a high hybrid proportion.

   Hence, the **7.5 %** bracket best fits the current situation.

6. **Feasibility within the issuance limit**  
   - Target hybrid equity credit of 7.5 % corresponds to a hybrid principal of ≈ 15 % of total adjusted capital (≈ €17 bn).  
   - With the 3 bn €/year issuance ceiling, the company can issue up to €6 bn in the 18‑month window, raising the hybrid principal to roughly €9‑10 bn (≈ 8‑9 % of total adjusted capital). This remains comfortably below the 15 % equity‑credit ceiling and moves the company toward the desired 7.5 % equity‑credit level.

**Conclusion** – To support refinancing, fund the high‑capex programme and optimise leverage while preserving rating headroom, Iberdrola should target a hybrid proportion that provides an equity credit of **7.5 %** of total adjusted capital.

<answer>7.5%</answer>