Let’s break down the reasoning:

1. **Company profile**: A2A Energia is an Italian utility, ultimately controlled by municipalities, with stable assets (PP&E and intangibles growing), regulated or quasi-regulated activities, and predictable cash flows.  
2. **Balance sheet risk**: Noncurrent liabilities grew from €5.5B to €7.2B, mostly from financial liabilities, showing increased leverage. Equity to total assets is around 21%, which is moderate for a utility, but not exceptionally strong.  
3. **Earnings & cash flow**: Revenue nearly doubled from 2021 to 2022, but operating profit grew only slightly, with a sharp rise in income tax expense eating into net profit. Cash flow from operations is positive and growing, but free cash flow turned barely positive in 2022 after being negative in 2021.  
4. **Market backdrop (2022)**: Interest rates moved sharply upward—5Y swap went from negative to +1.73%, 10Y to +1.93%. Corporate bond yields also rose materially. This increases the relative cost of issuing new debt, including hybrids.  
5. **Hybrid bonds**: These have equity-like features and can support credit ratings without full dilution. However, they are subordinated and come with higher coupons. Given A2A’s rising leverage, increasing financing costs, and only moderate free cash flow generation, a high reliance on hybrids would be risky and expensive. A moderate level (like 25%) could make sense to preserve credit metrics while avoiding excessive dilution.  
6. **Optimal choice**: 0% seems too conservative given growth investments and acquisitions; anything above 50% would raise the cost of capital too much in a rising-rate environment. 25% strikes the balance between capital structure optimization and cost management.

<answer>25%</answer>