Explanation:

- SBI has raised Rs 9,700 crore through its second round of infrastructure bonds.

- This move is notable as it marks the first time an Indian bank issues a specific term infrastructure bond.

- Option 1: 5-year – Common tenure for various bonds but not the first in this case.

- Option 2: 15-year – This is the correct answer and is significant for being an unusually long-term bond for Indian banks.

- Option 3: 8-year – This is a mid-range tenure, not making it unique for this context.

- Option 4: 12-year – Longer-term bonds exist, but not the first for this case.

- Option 5: 7-year – Another relatively short tenure not fitting the uniqueness here.