To determine the optimal extent to which Italgas S.p.A.'s capital structure should rely on hybrid bonds, we must analyze the company's financial profile, industry characteristics, and the specific benefits and limitations of hybrid securities.

1. **Industry Characteristics and Asset Profile**: Italgas operates in the gas distribution sector, a highly regulated utility industry. This is evidenced by the massive weight of intangible assets (primarily concession rights, totaling €8.5 billion) and property, plant, and equipment on its balance sheet. Regulated utilities typically have highly predictable and stable cash flows. This stability supports higher leverage, but also requires a capital structure that minimizes refinancing risk and aligns with long-lived assets.

2. **Current Capital Structure and Leverage**: Looking at the 2022 year-end data, the company has Total Equity of €2,390,567,000 and Total Liabilities of €8,639,987,000, resulting in a Debt-to-Equity ratio of approximately 3.6x. Within its liabilities, long-term financial debt stands at €6,402,913,000, while short-term financial debt is €142,437,000. Total financial debt is roughly €6.5 billion. The company is already highly leveraged.

3. **Interest Coverage and Cost of Debt**: The company's Profit Loss From Operating Activities for 2022 was €641,338,000, while its Finance Costs were €61,367,000. This implies an interest coverage ratio of around 10.4x, which is very healthy and indicates the company comfortably services its debt. However, with the swap curve and corporate bond spreads rising significantly in 2022 (e.g., 5-year swaps moving from -0.264% to 1.726%, and credit spreads widening), the cost of new senior unsecured debt is becoming substantial. 

4. **The Role of Hybrid Bonds**: Hybrid bonds (subordinated debt) possess equity-like features (e.g., optional deferral of interest, perpetual or very long maturity, deep subordination), which often allows rating agencies to treat 50% of the principal as "equity credit" when calculating leverage ratios. For a highly leveraged utility like Italgas, issuing hybrids is an efficient way to optimize the capital structure: it strengthens the equity cushion for senior creditors (potentially improving or maintaining credit ratings) while still providing a tax-deductible coupon (unlike common equity dividends).

5. **Quantifying the Optimal Extent (0%, 25%, 50%, 75%, 100%)**: 
    - **0%** is suboptimal because the company is already highly leveraged and rising interest rates make pure equity issuance dilutive and unattractive. Hybrids provide a necessary middle ground for leverage optimization.
    - **50%, 75%, or 100%** are excessively risky and impractical. Because hybrid interest is deferrable and the claims are deeply subordinated, relying too heavily on hybrids would drastically increase the company's overall cost of capital and signal financial distress. Rating agencies only grant partial equity credit (often 50%); thus, replacing the foundational senior debt with subordinated, equity-credit debt would invert the proper capital structure. Senior debt must remain the core of the liability stack to maintain low borrowing costs.
    - **25%** represents the ideal "sweet spot" for regulated utilities. A 25% hybrid weighting relative to total capitalization (or total debt) is a standard market practice for European utilities (such as Enel, Snam, or Terna) seeking to optimize their credit metrics without overpaying for capital. It provides sufficient equity credit to maintain a solid investment-grade rating amid high gross leverage, while keeping the majority of the debt stack in cheaper, senior form. 

Given Italgas's high leverage, stable utility cash flows, and the rising rate environment, a 25% allocation to hybrid bonds perfectly balances leverage optimization with capital cost efficiency.

<answer>25%</answer>