To determine the appropriate extent of hybrid bond usage in Veolia Environnement's capital structure, we must evaluate the company's financial profile based on the provided 2022 data:

1.  **Financial Leverage and Growth:** Veolia saw significant expansion in 2022, with "Noncurrent Financial Liabilities" increasing from approximately 10.5 billion EUR to 19.7 billion EUR. Simultaneously, "Equity Attributable To Owners Of Parent" grew from 11.5 billion EUR to 12.3 billion EUR.
2.  **Cost of Capital Environment:** The market data shows a sharp increase in interest rates between 2021 and 2022. The 10Y Swap rate jumped from 0.053% (2021) to 1.927% (2022), and the iBoxx EUR Non-Financial IG yield (a proxy for debt costs) increased from 1.298% to 2.295%.
3.  **Hybrid Bond Role:** Hybrid securities (classified here as "Titres Super Subordonnés A Durée Indéterminée") increased from 2.46 billion EUR in 2022 to 3.50 billion EUR in 2023. These instruments act as a buffer, providing equity-like characteristics (supporting credit ratings) while maintaining leverage flexibility.
4.  **Strategic Balance:** Given the rising interest rate environment, hybrid bonds are an efficient tool for Veolia to manage its credit rating while funding the capital-intensive infrastructure projects characteristic of their sector. However, relying too heavily on hybrids (e.g., >50%) would increase the weighted average cost of capital due to the higher coupons required compared to senior debt. A 25% allocation represents a prudent balance that supports the company’s capital-intensive nature (as evidenced by high PPE and concession assets) without over-burdening the income statement with excessive coupon payments, especially considering the "Coupons On Deeply Subordinated Securities" expense rose from 23.9M EUR to 88.3M EUR.

<answer>25%</answer>