Explanation:

• Option A (Mezzanine debt): Incorrect. Mezzanine debt is a junior form of subordinated debt, offering higher growth potential and equity upside but also carrying higher risk compared to senior debt.

• Option B (Unitranche debt): Incorrect. Unitranche debt combines features of senior and subordinated debt, typically ranking between them in priority. While it may offer a balance, it still carries more risk than senior infrastructure debt.

• Option C (Infrastructure debt): Correct. Infrastructure debt is senior and secured, often backed by stable cash flows from essential projects, making it the least risky among the options. It provides lower returns but also lower risk compared to mezzanine and unitranche debt.

This question tests understanding of private debt structures and their associated risk profiles.