To determine the extent to which ENEL S.p.A. should utilize hybrid bonds relative to the S&P Global Ratings' 15% equity credit cap, we first calculate the total adjusted capital and the existing hybrid bond position.

1.  **Adjusted Capital Calculation (as of 2023-01-01):**
    *   **Equity Attributable to Owners of Parent:** 28,657,000,000 EUR
    *   **Noncontrolling Interests:** 13,425,000,000 EUR
    *   **Total Equity:** 42,082,000,000 EUR
    *   **Long-term Borrowings:** 68,191,000,000 EUR
    *   **Short-term Borrowings:** 18,392,000,000 EUR
    *   **Total Debt (Adjusted):** 86,583,000,000 EUR
    *   **Total Adjusted Capital (Debt + Equity):** 128,665,000,000 EUR

2.  **S&P Hybrid Cap Calculation:**
    *   The S&P limit for equity credit on hybrids is 15% of the total adjusted capital.
    *   15% of 128,665,000,000 EUR = 19,299,750,000 EUR.

3.  **Existing Hybrid Position:**
    *   From the equity data, the "Equity Instruments Perpetual Hybrid Bonds Member" as of 2023-01-01 is 5,567,000,000 EUR.

4.  **Utilization Ratio:**
    *   Current utilization relative to the S&P cap: 5,567,000,000 / 19,299,750,000 ≈ 28.8%.

5.  **Reasoning:**
    *   The company is currently utilizing approximately 29% of its available S&P equity credit capacity for hybrid bonds. Among the provided options (0%, 25%, 50%, 75%, 100%), 25% is the closest representation of the company's current conservative stance relative to the maximum allowable limit. Increasing usage beyond current levels significantly would heighten financial risk and potential interest expense volatility given the current interest rate environment (as observed in the rising swap curves from 2021 to 2022). Therefore, 25% is the most appropriate target for efficient capital structure management within the stated regulatory constraints.

<answer>25%</answer>