To determine the appropriate extent to which Italgas S.p.A. should rely on hybrid bonds, we must evaluate its financial stability, debt profile, and cost of capital environment.

1.  **Debt Profile:** Italgas maintains a significant amount of long-term financial debt (6,402,913,000 EUR in 2023) compared to total equity (2,390,570,000 EUR). This high leverage ratio indicates that the company is capital-intensive and relies heavily on debt financing.
2.  **Interest Rate Environment:** The provided market data shows a sharp increase in swap curves (e.g., the 10Y swap curve rose from 0.053% in 2021 to 1.927% in 2022). As interest rates rise, refinancing existing debt becomes more expensive. Hybrid bonds, which provide equity-like credit treatment (often 50% equity credit from rating agencies) while maintaining tax-deductible interest payments, can be an efficient tool to manage leverage and credit ratings during periods of rising rates.
3.  **Financial Health:** The company has robust cash flows from operations (548,169,000 EUR in 2023) and generates consistent revenue (2,312,476,000 EUR). This cash-generating capability supports the ability to service more expensive hybrid debt.
4.  **Strategic Balance:** Relying on 0% hybrid debt ignores the benefit of optimizing the balance sheet to support credit ratings. Conversely, 75% or 100% would be excessively expensive and unnecessary, as the company has no indicated liquidity crisis. A moderate approach, utilizing hybrid instruments for a portion of the long-term funding mix, is standard for utilities with significant capital expenditure requirements. A 25% allocation of hybrid capital in the debt structure allows for credit rating support without excessive interest burden, balancing the cost-efficiency of debt with the rating benefits of equity.

<answer>25%</answer>