To determine the extent to which ELECTRICITE DE FRANCE (EDF) should utilize hybrid bonds relative to S&P Global Ratings' 15% cap on total adjusted capital, we assess the cost-benefit of hybrids in the capital structure. 

1. **Hierarchy of Cost of Capital:** Hybrid bonds sit between senior debt and common equity in the capital structure. They are riskier for investors than senior debt but less risky than equity. Consequently, the required return (or cost) of hybrid bonds is higher than that of senior debt but lower than the cost of equity ($K_d < K_h < K_e$). 
2. **Tax Shield Advantage:** Unlike common equity dividends, the coupon payments on hybrid bonds are generally tax-deductible, bringing their after-tax cost down further compared to the cost of equity. 
3. **Rating Agency Treatment:** S&P Global Ratings treats hybrid bonds as having "equity-like" characteristics (giving them equity credit) up to a limit of 15% of the total adjusted capital (Total Debt + Equity). Above this cap, hybrids are treated as standard debt, removing their rating-agency advantage of being a cushion against distress.

Because hybrid bonds are strictly cheaper than equity while offering the same rating-agency benefits up to the 15% threshold, a firm aiming to minimize its Weighted Average Cost of Capital (WACC) and optimize its capital structure should always replace equity with hybrid bonds up to that exact limit. Given these fundamental corporate finance mechanics, EDF should fully utilize this allowable bucket.

<answer>100%</answer>