To determine the extent to which A2A should utilize hybrid bonds relative to the S&P Global Ratings' equity credit cap, we first need to calculate the company's Total Adjusted Capital and the maximum amount of hybrid bonds that would receive equity credit.

1. **Calculate Total Adjusted Capital:**
Total Adjusted Capital is defined by S&P as Debt plus Equity. 
From the balance sheet data:
- Equity (2023-01-01) = 4,467,000,000 EUR
- Liabilities (2023-01-01) = 16,900,000,000 EUR
Total Adjusted Capital = Equity + Liabilities = 4,467,000,000 + 16,900,000,000 = 21,367,000,000 EUR.

2. **Calculate the S&P Equity Credit Cap:**
S&P limits the equity credit on hybrids to a maximum of 15% of Total Adjusted Capital.
Maximum Equity Credit Cap = 15% × 21,367,000,000 = 3,205,050,000 EUR.

3. **Determine Existing Hybrids / Subordinated Debt:**
We must check how many hybrid bonds the company already has issued. Under "Noncurrent Liabilities", we see "Other Noncurrent Financial Liabilities" of 5,867,000,000 EUR. However, looking at the cash flow statement, the breakdown of financing activities shows "Proceeds From Borrowings", "Repayments Of Borrowings", and "Payments Of Lease Liabilities", with no specific line item for issuances or repayments of hybrid/subordinated capital. More specifically, the Statement of Changes in Equity shows the breakdown of equity and reserves, and there are no hybrid instruments classified as equity (like mandatory convertible instruments or similar). Given the structure of A2A's balance sheet and disclosures, the existing equity-credit-worthy hybrid bonds are effectively 0 EUR (or negligible relative to the cap). 

4. **Evaluate the Extent of Utilization:**
With 0 EUR of existing hybrids against a cap of 3.2 billion EUR, the company is currently utilizing 0% of S&P's equity credit capacity. Therefore, relative to the cap, the company can utilize up to 100% of the allowable limit.

5. **Market Data Context:**
Interest rates rose significantly in 2022 (as shown by the Swap Curves and Corporate Bond yields), making the issuance of fixed-rate hybrids more expensive. However, the question asks specifically to what extent the company *should* utilize hybrids *relative to S&P's cap*. Since they are at 0% utilization, the maximum relative extent they can and should consider to optimize their capital structure and leverage metrics (while retaining equity credit) is the full 100% of the allowable cap.

<answer>100%</answer>