To determine the optimal extent to which Acciona SA's capital structure should rely on hybrid bonds, we evaluate the cost of capital associated with different funding sources. 

Hybrid bonds typically sit between senior debt and equity in the capital structure. They carry a higher cost than senior debt due to their subordinated nature and the additional risks borne by investors (as indicated by the Sub-Sen Delta, representing the yield premium of subordinated/hybrid debt over senior debt). For 2022, the proxy for standard corporate bond yields (such as the iShares Core Euro Corp Bond) averaged 1.085%, while the sub-senior delta averaged an additional 2.295%. 

Because standard senior debt is strictly cheaper than hybrid debt ($Cost\ of\ Senior\ Debt < Cost\ of\ Hybrid\ Debt < Cost\ of\ Equity$), a standard WACC (Weighted Average Cost of Capital) minimization approach without strict constraints on credit rating thresholds or financial distress costs will mathematically favor senior debt over hybrid debt. Thus, to minimize the overall cost of capital, the theoretical optimal allocation to hybrid bonds is 0%, with the firm instead maximizing its use of cheaper standard senior debt before relying on equity. 

Furthermore, from a practical standpoint, no non-financial corporation employs hybrid bonds for 25% or more of its entire capital structure, as rating agencies strictly limit the amount of hybrid debt that can receive partial equity treatment (usually capping it at 10% to 15% of total capitalization). 

Therefore, the optimal reliance on hybrid bonds among the given choices is 0%.

<answer>0%</answer>