Let’s reason through this step-by-step.

**1. Business and asset profile**  
Acciona SA is an infrastructure and renewable energy company with significant amounts of long-lived assets (high Property, Plant & Equipment, investment in associates, etc.). This implies a need for stable, long-term financing that matches asset duration.

**2. Capital structure review**  
Looking at the balance sheet:
- Equity as of 2023-01-01 is €6,304M versus total assets of €22,595M, for an equity ratio of ~28%.  
- Non-current liabilities are €8,467M.
- The company already uses significant debt, including loans and securities, and also has substantial minority interests.

**3. Hybrid bond suitability**  
Hybrid bonds are treated partly as equity by rating agencies and can strengthen the credit profile without diluting shareholders. For a capital-intensive group like Acciona, some hybrid utilization helps maintain leverage ratios while preserving strategic flexibility. However, 100% hybrid would be unrealistic due to cost and investor appetite. Likewise, 0% ignores the benefits of equity credit and cost-efficiency hybrids provide.

**4. Market conditions (2022)**  
Rates increased sharply in 2022 (swap curves rose), and credit spreads widened moderately. This makes pure debt more expensive, but hybrid bonds, being subordinated and longer-dated, are even more rate-sensitive. Still, IG non-financial spreads remain manageable.

**5. Reasonable hybrid proportion**  
For a firm of this profile, a moderate proportion like 25% of subordinated/hybrid instruments within total debt capital would be plausible—enough to get rating agency equity credit, but not so much as to incur excessive interest costs or investor concentration limits.

<answer>25%</answer>