**Reasoning**

When selecting the most promising issuers for a new hybrid‑bond transaction, a bank will look at several factors:

1. **Existing hybrid program** – Issuers that already have hybrid capital in place are more familiar with the instrument and more likely to issue again in the near term. Both ENEL (C) and TenneT (A) already report hybrid bonds (5.6 bn EUR and 2.1 bn EUR respectively), while ENI (B) has none.

2. **Scale of the potential deal** – Larger companies can support bigger hybrid issues, generating higher fees. ENEL (revenue ~140 bn EUR, assets ~220 bn EUR) and ENI (revenue ~133 bn EUR, assets ~152 bn EUR) are far larger than TenneT (revenue ~8 bn EUR, assets ~39 bn EUR). ENEL therefore offers the greatest deal size.

3. **Credit quality and cash‑flow strength** – ENEL’s leverage is high (≈5×) but it already uses hybrid bonds to manage its capital structure and has solid operating cash flows (≈8.7 bn EUR). TenneT is regulated and state‑owned, provides stable cash flows (operating cash flow ≈1.2 bn EUR) despite a net loss, and already relies on hybrid financing. ENI is highly profitable (net profit ≈14 bn EUR) with low leverage, making it a high‑quality borrower but with less immediate need for a hybrid issuance.

4. **Strategic need and market readiness** – ENEL is actively expanding its hybrid program and is a known hybrid issuer, making it the most immediate prospect. TenneT, as a regulated transmission‑system operator with a government owner, also has a clear regulatory incentive to maintain equity‑like capital and is likely to issue further hybrid notes. ENI, while credit‑worthy, has no current hybrid program and would require more convincing, making it a lower priority.

Putting these considerations together, the most promising prospect is **ENEL (C)**, followed by **TenneT (A)**, and then **ENI (B)**.

<answer>C,A,B</answer>