First, let’s reason through the key points.

**1. Nature of the business**  
REN is a Portuguese energy transmission network operator. This means it has monopoly-type, regulated, stable cash flows. Such companies tend to be highly leveraged with senior debt but may use hybrids to protect credit ratings while raising capital.

**2. Current capital structure**  
From the 2023-01-01 data:  
- Equity: €1.518 billion  
- Long-term borrowings: €1.695 billion  
- Current borrowings: €0.639 billion  
Total debt ≈ €2.33 billion.  
Debt-to-equity ≈ 1.54, which is moderately high but normal for a regulated utility.

**3. Earnings and cash flow stability**  
Revenue and operating income are steady (~€825 million in 2022). Operating cash flow is strong (€613 million). The stable regulatory framework supports reliable interest payments, which is good for hybrids.

**4. Market conditions**  
Swap rates rose substantially in 2022, and corporate spreads widened. This means hybrid issuance costs would be higher than in prior years, but still possibly beneficial to avoid a downgrade from additional senior debt.

**5. Hybrid bond rationale**  
Hybrids (part debt, part equity treatment by rating agencies) can strengthen the balance sheet without full equity dilution. Given REN’s significant capex needs (intangible assets additions €201 million), hybrids could help finance growth while keeping leverage ratios in check.

**6. Extent of reliance**  
100% reliance would be too risky—utilities need some plain debt. 0% ignores the rating benefit. A moderate portion such as 25% of total capital structure in hybrids would balance cost, rating stability, and financial flexibility.

Given this, a **25%** reliance on hybrid bonds seems reasonable for REN.

<answer>25%</answer>