A restaurant bill of ₹3,000 already includes a service charge of 25% that was added on to the food value. The service charge is what per cent of the ₹3,000 bill?

The 25% was added TO the food value, so the food value $F$ satisfies $1.25F=3000$, giving $F$ = ₹2,400 and a service charge of ₹600. As a share of the bill that is 600/3000 = 20%. 25% is the star trap: that rate belongs to the smaller base (the food value), not to the bill. If you wrongly take the charge as 25% OF the bill you get ₹750 and a food value of 75% - hence those two options. 80% is the food value's true share of the bill.