To determine the optimal extent to which BOUYGUES should utilize hybrid bonds relative to S&P Global Ratings' cap, we must evaluate the cost of capital dynamics between hybrid bonds and pure equity.

1. **Nature of Hybrid Bonds**: Hybrid bonds blend characteristics of both debt and equity. For the issuer, they are subordinated to senior debt but sit senior to common equity. Because they carry a fixed or semi-fixed coupon that is typically tax-deductible (unlike equity dividends), they have a distinctly lower cost of capital than common equity.
2. **Equity Credit**: S&P Global Ratings and other major rating agencies typically grant "equity credit" (usually 50%) for hybrid bonds, meaning they treat half of the hybrid bond's value as equity and half as debt when calculating credit metrics. This allows a company to support its credit rating without diluting existing shareholders or bearing the higher cost of common equity.
3. **The S&P Cap**: S&P limits the amount of hybrid capital that can receive this equity credit to 15% of Total Adjusted Capital (TAC), which is defined as total debt plus equity. Beyond this 15% cap, any additional hybrid bonds issued are generally treated as 100% debt, stripping away their rating benefits while remaining more expensive than senior debt.
4. **Optimization Strategy**: Because hybrid bonds are strictly cheaper than pure equity (due to seniority and tax shields) yet provide equity-like rating benefits up to the 15% cap, a rational firm seeking to minimize its Weighted Average Cost of Capital (WACC) while maintaining its credit profile will maximize its use of hybrid bonds up to this exact limit. 

Since there are no overriding distressed financial conditions for BOUYGUES (the firm generates strong recurring operating profits and possesses significant retained earnings), the optimal financial strategy is to fully exhaust the 15% limit. 

Therefore, the company should utilize 100% of the allowable S&P cap for hybrid bonds.

<answer>100%</answer>