Kay, I assume the answer was 'sur' ?
The use of en, à , par relates to “travelling” BY/IN the mode of transport - you can still be in, get in or be under a car, sit on or stand beside a bike etc for other purposes, with the appropriate French prepositions used in those contexts.
Although not the only trap, it is important to remember though that in French, 'sur' is not used for travelling 'by' or 'in' a mode of transport. If you say you travelled 'sur l'avion' or 'sur le train' there will be much French bemusement, to say the least !
À/en/par + [means of transportation] (French Prepositions)
I'm not surprised that you are having difficulty with this issue.
Some sites that I have used suggest that "en" is used for a mode of enclosed transport that one gets into -- bus / car / boat / aircraft etc.
Others suggest using "en" for being on a bike.
I can't see why "sur" is not appropriate for a bike -- I would instinctively opt for that preposition, the rider is actually "on" the machine.
I have however seen the term "en velo"
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