To determine the optimal reliance on hybrid bonds for ENGIE's capital structure, we must evaluate the cost-effectiveness of this financing tool given the prevailing market conditions and corporate finance principles. 

Hybrid bonds, such as deeply subordinated perpetual notes, blend characteristics of both debt and equity. While they are often issued by non-financial corporates to defend their credit ratings (as rating agencies generally attribute around 50% equity credit to them), they come at a higher cost than senior debt. 

Looking at the provided market data for 2022:
1. **Cost of Senior Debt**: The average yield for the iShares Core Euro Corp Bond (a proxy for Euro-denominated investment-grade corporate bonds) is 1.085%.
2. **Subordinated Premium (Sub-Sen Delta)**: The yield premium required by investors to hold subordinated debt over senior debt for iBoxx EUR Non-Financial IG is 2.295%.
3. **Cost of Hybrid Debt**: Adding the premium to the base yield, the approximate cost of hybrid/subordinated debt is 1.085% + 2.295% = 3.38%.

Given that the Sub-Sen Delta represents a very steep premium (more than double the base investment-grade yield), issuing hybrid bonds is highly expensive compared to traditional senior debt. In pure corporate finance theory—absent strict rating agency equity-credit threshold constraints (which typically cap hybrids at 15% of total capital regardless)—a company should optimize its Weighted Average Cost of Capital (WACC) primarily through an optimal mix of pure equity and senior debt. Hybrids are generally an inefficient, frictional source of capital mathematically because the yield penalty is typically too high for the tax shields they provide. 

Additionally, examining ENGIE's balance sheet, its total Deeply Subordinated Perpetual Notes at the end of 2022 stood at approximately €3.39 billion against total equity of €39.28 billion and total assets of €235.49 billion. This indicates that hybrids realistically make up a low-single-digit percentage of their overall capital structure (~4%). Therefore, out of the quartile options provided, the closest and most theoretically sound target is 0%.

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