The core test for a legal set-off is whether both the plaintiff's claim and the defendant's counter-claim are for a fixed, ascertained sum of money.
Only the option describing an ascertained, recoverable sum satisfies both conditions, so the correct answer is Suit for recovery of ascertained sum of money.
Set-off exists to avoid multiplicity of suits by letting a defendant adjust a cross-debt within the same proceeding, and this purpose explains why the law insists the amount be fixed and recoverable.
Only the ascertained, legally recoverable sum serves the purpose set-off is designed for.
Therefore, the correct answer is Suit for recovery of ascertained sum of money.