Reasoning:

Terna is a regulated infrastructure utility with stable cash flows, high asset intensity, and large ongoing investment needs. These characteristics generally make hybrid bonds useful because they can support credit metrics while funding capex without issuing ordinary equity.

Key points from the 2022 data:

- Terna already issued a sizeable perpetual hybrid bond in 2022: €989 million.
- Total equity at year-end 2022 was €6.17 billion, helped by the hybrid instrument.
- Reported borrowings were substantial:
  - Long-term borrowings: €8.42 billion
  - Short-term borrowings: €0.44 billion
  - Current portion of long-term borrowings: €1.91 billion
  - Total borrowings: about €10.77 billion
- Approximate total adjusted capital, using debt plus equity, is therefore around €16.94 billion.
- S&P’s 15% hybrid equity-credit cap would imply a maximum equity-creditable hybrid amount of about:
  - 15% × €16.94 billion ≈ €2.54 billion
- The current hybrid amount of €989 million is roughly:
  - €989 million / €2.54 billion ≈ 39% of the cap

So Terna is already using hybrids meaningfully, but not close to the S&P cap. The company has strong operating cash flow (€2.32 billion), profitable regulated operations, and improving equity, but also high leverage and major investment requirements. Market rates rose sharply in 2022, making further hybrid issuance more expensive than in prior years. Therefore, fully maximizing the cap would likely be too aggressive, but moderate additional use is justified.

Among the available choices, the current usage is closest to 50% of the cap, and that also fits the qualitative assessment: hybrids should be used as a meaningful but not maximized financing tool.

<answer>50%</answer>