To determine the appropriate reliance on hybrid bonds for REDEIA CORPORACION SA, we analyze its financial structure, credit profile, and industry norms:

1. **Credit Profile and Industry**: Redeia is a Spanish regulated utility (electricity transmission). Such companies typically have stable, predictable cash flows and very high credit ratings (Investment Grade, likely A- or higher). Hybrid bonds are deeply subordinated, have long maturities, and often include interest deferral features. They are typically used by very strong credits (A-rated or above) because the market must trust that the company will not exercise the deferral option. Redeia fits this profile perfectly, making it an ideal candidate for hybrid bonds, which are widely used in the European utility sector.

2. **Capital Structure Analysis**: 
   - Total Capital = Equity + Noncurrent Financial Liabilities + Current Financial Liabilities = 4,894,276,000 + 5,543,755,000 + 1,705,277,000 = 12,143,308,000 EUR.
   - Traditional Equity (Equity + Noncontrolling Interests) = 4,894,276,000 EUR (40.3% of total capital).
   - Total Debt = 5,543,755,000 + 1,705,277,000 = 7,249,032,000 EUR (59.7% of total capital).
   - Given that hybrid bonds receive 50% equity credit from rating agencies (like S&P and Fitch) up to a limit (typically the "hybrid basket" allows hybrids to constitute up to the lesser of total traditional equity or a fixed percentage of total capital, usually capped around 25% of total capital for A-rated utilities), we assess the hybrid capacity.
   - To maintain a solid A-level rating, a utility generally targets a total adjusted equity (Traditional Equity + 50% of Hybrids) of around 50% of total capital. 
   - Currently, traditional equity is roughly 40%. If hybrids were 25% of the total capital (~3,036M EUR), total adjusted equity would become 40.3% + (50% × 25%) = 52.8%, which is a very comfortable A-range adjusted capitalization for a utility.
   - If hybrids were 50% of capital, total adjusted equity would be 40.3% + 25% = 65.3%, which is excessively over-equitized and expensive (hybrids yield more than senior debt). If 75% or 100%, it would be absurd for a utility. If 0%, the firm is missing a key tax-efficient, rating-agency-approved optimization tool heavily utilized by its European peers.

3. **Conclusion**: European regulated utilities commonly optimize their capital structures by issuing hybrid bonds up to their agency-approved limit, which effectively maxes out around 20-25% of their total capitalization. Therefore, Redeia should rely on hybrid bonds for 25% of its capital structure.

<answer>25%</answer>