cycling is a good exercise. mr. mehta prefers cycling.....driving a car.
@ nagpal.pankaj1100, your friend put in good effort in conveying the right answer to you. However, the correct explanation for the same is:
Prefer always comes with “to” when you want to highlight your preference or priority with reference to a lesser important choice.
1. I prefer pineapple juice to guava juice;
2. They prefer Ajmer to Agra for their excursion trip; etc.
Thus, the correct word in the blank of the sentence you asked for is “to”.
‘Cycling is a good exercise. Mr. Mehta prefers cycling to driving a car.’
@Shobitha, keep up the spirit of helping!
Good Luck to all!