To determine the appropriate reliance on hybrid bonds for A2A's capital structure, we need to evaluate the firm's financial profile, credit characteristics, and the strategic role of hybrid securities.

1. **Company Profile & Credit Standing**: A2A S.p.A. is a large Italian multi-utility entity (electricity, gas, water, waste management). Utilities are characterized by stable, regulated cash flows, high capital expenditures, and asset-heavy balance sheets. Looking at the 2022 financials, A2A has an Equity/Assets ratio of ~21% and a Net Financial Position (Financial Debt - Cash; calculated as Noncurrent Financial Liabs + Current Financial Liabs - Cash & Equivalents) of roughly €4.8 billion. The credit spread for non-financial European investment-grade corporates (proxied by the iBoxx data and sub-sen delta) sits around 1.0%–2.3%, confirming it is a solid IG credit. 

2. **Role of Hybrid Bonds**: For IG utilities, hybrid bonds are a standard and highly efficient tool. They are deeply subordinated and perpetual (or with long call periods), allowing rating agencies (like S&P, Moody's, and Fitch) to treat them as 50% equity and 50% debt. This provides "equity credit" which bolsters the credit rating and strengthens financial covenants (like FFO/Debt), while avoiding the dilution and higher cost of pure equity. However, they do carry mandatory coupon payments (lack of tax-deductible dividend flexibility) and increase the weighted cost of capital if over-used. 

3. **Appropriate Reliance**: While hybrid bonds are standard for European utilities to optimize their capital structure and maintain solid IG ratings, they cannot form the majority or even a large plurality of the capital structure. The " basket limit" (the maximum amount of hybrids rating agencies will treat as equity before downgrading the hybrids themselves or ceasing to give equity credit) typically maxes out at around 15% to 25% of the total adjusted capitalization for utilities. A reliance of 25% aggressively optimizes the structure, pushing the limits of agency basket limits while protecting the core IG rating. Anything above 25% (50%, 75%, 100%) would vastly exceed equity credit baskets, heavily burden the firm with fixed coupon obligations, and destroy the financial flexibility that makes hybrids attractive in the first place. 0% is suboptimal as it ignores the proven financial engineering benefits available to this specific sector.

Therefore, the maximum optimal extent to which a company like A2A should rely on hybrid bonds in its capital structure is 25%.

<answer>25%</answer>