**Reasoning**

Bouygues is a large, investment‑grade conglomerate with a capital base of roughly €30 bn (equity ≈ €13.9 bn, total debt ≈ €15.6 bn). Its net debt rose sharply in 2022 (from €0.9 bn to €7.4 bn) largely to finance acquisitions, yet interest coverage remains comfortable (EBIT/interest ≈ 6×) and operating cash flows are strong (≈ €3 bn after tax and interest).  

In the current market environment the risk‑free swap rates have turned positive (≈ 1.7‑2 % for 5‑10 Y) and investment‑grade corporate bond yields are around 1‑2 % above the risk‑free rate, implying senior debt can be issued at roughly 2‑3 %. Hybrid bonds typically price 100‑200 bps above senior paper, making them more expensive than plain senior financing.  

Nevertheless, hybrid instruments are treated as partially equity by rating agencies (≈ 50 % equity credit), which can improve leverage metrics and help maintain the group’s BBB rating while it continues to absorb large goodwill and intangible assets from acquisitions. Given the firm’s already substantial debt load, a modest proportion of hybrid financing can provide the equity‑like buffer needed without over‑burdening the cost of capital.

Therefore, a limited reliance on hybrid bonds—enough to capture the rating benefit while avoiding the premium cost of a larger hybrid tranche—is appropriate. A share of roughly one‑quarter of total debt (≈ 25 % of the capital structure) strikes the right balance.

**Final answer**

<answer>25%</answer>