a. が b. で c. に d. を
The most common answer is c. に: I ride a bike.
のる means to get on something (and be carried by it) and it is an intransitive verb. So のる is never used with d. を, which you need to remember as “to ride (a bicycle)” in English is a transitive verb.
Other usages of 乗（の）る include:
車（くるま）に 乗（の）る – to get in a car (likely to be as a passenger but could be a driver also)
調子（ちょうし）に 乗（の）る – to be carried away
相談（そうだん）に 乗（の）る – to give advice
I said c. に is the most common answer.
If you use a. が, じてんしゃが のる can be a sentence and it means “A bicycle is on (a train or something).” As a bicycle itself cannot get onboard, じてんしゃを のせる is the more common way of saying it but じてんしゃが のる is not grammatically wrong.
Using b. で, じてんしゃで のる is not grammatically wrong either. It can be used if/when you are using a ferry or similar with your bicycle.