Why do you travel by . . . . . bus?

Using "a" and "the" depends on how specific or general we want to be when talking about something.

"A" is used for a non-specific or generic reference. For example, "I travel by a bus" implies any bus in general.

"The" is used when referring to a specific instance or when both the speaker and the listener know which particular bus is being discussed. For instance, "I travel by the bus" could imply a specific bus service or route that is commonly known between the speaker and the listener.

In the above question "Why do you travel by a bus" is more appropriate because it is a general talk rather than any specific bus service or route.

Therefore, option A is the correct answer here.