To determine the appropriate extent to which Eni S.p.A.'s capital structure should rely on hybrid bonds, we must analyze the company's financial profile, the role of hybrid instruments, and market conditions.

1. **Role of Hybrid Bonds**: Hybrid bonds (such as perpetual subordinated bonds) possess both debt-like characteristics (coupon payments, tax deductibility) and equity-like characteristics (perpetual maturity, subordination, optional deferral of coupons). Rating agencies typically treat qualifying hybrids as 50% equity, making them an efficient tool for companies to maintain a strong credit rating while optimizing their cost of capital and return on equity.

2. **Company's Financial Position (End of 2022)**: Eni is a large, integrated oil and gas major. At the end of 2022, its total assets stood at €152.13 billion, with equity of €55.23 billion and total liabilities of €96.9 billion. The long-term borrowings were €19.37 billion, and short-term borrowings plus the current portion of long-term debt amounted to roughly €7.54 billion. The company already utilizes hybrid capital, as evidenced by the "Increase Decrease Through Net Issue Reimbursement Of Perpetual Subordinated Bonds Equity" and "Decrease Through Coupon Payment On Perpetual Subordinated Bonds Equity" line items in the equity changes. The nominal outstanding perpetual subordinated bonds account for roughly €2-3 billion (indicated by the €2B net issue in 2021, €3B in 2020, and other movements), which represents approximately 5-10% of its total capitalization (Equity + Financial Debt).

3. **Capital Structure Strategy**: For a cyclical, capital-intensive, and commodity-exposed business like Eni, financial flexibility and maintaining an investment-grade rating are paramount. A 25% hybrid bond reliance in the total capital structure (Equity + Net Debt) would imply roughly €15-20 billion of hybrid bonds, a level that would excessively subordinate traditional creditors, significantly increase the effective cost of debt, and likely breach the "limited" hybrid allowance criteria set by rating agencies (which generally cap hybrid recognition at around 15-25% of total equity rather than total capital). A 0% reliance ignores the distinct tax and rating agency benefits of hybrids, particularly when the company already actively issues them. 

4. **Market Conditions**: The 2022 swap curves and corporate bond spreads were normalizing and rising, reflecting higher interest rates. While hybrids offer a relatively cheap source of equity credit for rating agencies, over-reliance on them in a rising rate environment exposes the company to significantly higher refinancing and coupon step-up risks. 

Therefore, while hybrids should be a meaningful component of the capital structure to optimize the weighted average cost of capital and protect the credit rating, their inherent risks and rating agency limits restrict their use to a moderate, supplementary role. A reliance of 25% is the most appropriate level among the options to represent a notable but limited and prudent utilization of hybrid instruments in a large energy company's capital structure.

<answer>25%</answer>