Let’s go through this step-by-step.

**1. Business and industry profile**
- ACEA S.p.A. is an Italian multi-utility, operating primarily in water, electricity, and gas distribution, as well as energy sales and environmental services.
- The Italian regulatory framework for utilities is historically fairly stable but has moderate political interference risk. It would likely fall into the “adequate” or “adequate/strong” regulatory advantage range.
- Overall business risk is moderate, and given the regulated/essential nature of the services, the company likely qualifies for medial volatility under S&P’s methodology.

**2. Financial position (based on 2022 financials)**
- Revenue grew to €5.14 billion (from €3.97 billion), driven mainly by higher energy prices.
- Net profit attributable to owners of parent declined to €279.7 million (2021: €313.3 million).
- Operating cash flow remained solid at ~€727 million.
- Capex was heavy: ~€350 million tangible + €700 million intangible → total investing activities net outflow ~€863 million.
- Total equity at end-2022 was €2.755 billion. Adjusted total capital will be higher once debt is added.
- Reported net debt is not directly given, but long-term financial liabilities alone are ~€4.72 billion, so leverage is meaningful.
- FFO-to-debt likely moderate; interest costs rose to €111.7 million (2021: €97.4 million) with rising rates.
- The company already has some hybrid-like characteristics (significant minority interests, complex financing), but no explicit hybrids are referenced in the data.

**3. Rationale for hybrid usage**
- Acea has substantial capex plans (as seen in cash flows), likely driven by energy transition/infrastructure renewal.
- With rising interest rates (5Y swap from negative to ~1.73% in 2022, 10Y ~1.93%), incremental senior debt cost is increasing; hybrid cost would be higher but possibly manageable.
- Leverage is already elevated, and continuing high capex could pressure credit metrics without equity-like support.
- No current hybrids in the capital structure, so starting point is 0%.
- The case is for moderate usage: the company would benefit from some equity credit to maintain rating headroom, but it is not in a transformational M&A or distress scenario.
- Cost of hybrid would likely increase the current average cost of debt, making a full 15% allocation excessive.

**4. Matching to guideline ranges**
- Moderate funding needs, mild leverage optimization, some rating flexibility → **3.75%–7.5% range**.
- Given the scale of capex and some leverage pressure, a stronger case can be made for **7.5%** than just 3.75%.
- 11.25% would be justified only if there were high M&A or very constrained headroom, which is not evident here.
- 15% is too aggressive—cost of hybrid would clearly exceed current debt cost and there is no rating-crisis scenario.

**Conclusion**
The most balanced recommendation is **7.5%** of total adjusted capital, allowing meaningful support for leverage metrics while keeping incremental cost manageable.

<answer>7.5%</answer>