Let’s break this down step-by-step.

**1. Nature of the business**  
ACEA S.p.A. is an Italian multi-utility company — mainly regulated or quasi-regulated infrastructure (water, electricity distribution, waste management). Such companies have predictable, stable cash flows and can sustain higher leverage than cyclical industrials.

**2. Current capital structure**  
From the equity section:  
- Equity attributable to owners of parent (end 2022): ~€2.29 bn  
- Non-controlling interests: ~€0.46 bn  
- Total equity: ~€2.76 bn  
- Non-current financial liabilities: ~€4.72 bn  
- Current financial liabilities: ~€0.62 bn  
So it is already quite leveraged.

**3. Profitability and coverage**  
- EBIT (2022): ~€566 mn  
- Finance costs: ~€112 mn  
- Profit before tax: ~€498 mn  
Interest cover (EBIT / finance costs) ≈ 5x — reasonable but not excessive headroom.

**4. Market conditions (2022)**  
Interest rates rose sharply in 2022 — swap rates jumped from negative to ~1.7–1.9%. Corporate bond yields also increased. In such an environment, adding subordinated or deeply subordinated (hybrid) debt is more expensive, and rating agencies may give limited equity credit if coverage weakens.

**5. Hybrid bond suitability**  
Hybrid bonds are subordinated, have deferrable coupons, and get partial (50%) equity credit from rating agencies. They suit:  
- Companies seeking to maintain investment-grade rating  
- Those with stable regulated cash flows  
- Cases where shareholder structure limits equity issuance  

For ACEA, some hybrid debt could help fund its large capex program without diluting shareholders or risking a downgrade. However, 100% or 75% reliance on hybrids is impractical — senior debt remains cheaper and the company benefits from a mix.

Given its already significant leverage and the rising rate cycle, **a moderate use of hybrids**, around 25% of new debt or capital structure, would be sensible to preserve rating headroom and cost efficiency.

<answer>25%</answer>