Do you come to school by bus or (by) car?

Is 'by' obligatory?

Hi Yoong,
I don't know if it is "obbligatory" or not... However, I think it definitely sounds better if you leave that "by" there.
I have a feeling that it is always better not to leave out prepositions or articles in sentences like that. Maybe you could get a shorter sentence, but often you could get an odd one.
I also tried to think of your sentence this way:

Do you come to school by bus? Or (by) car?

If I split it that way, leaving out "by" seems even worse to me now.

So I'll leave it there in any case, but that's my opinion. We'd better wait for some native speakers and see what they think. Emotion: smile
Is 'by' obligatory?
No. Absolutely not.

Do you come to school by bus or car? [completely normal and fine]

A preposition can take a compound object as long as the general meaning stays the same. That is, you cannot compound the prepositional objects in a sentence thus:

*She was in a car and a bad mood. Emotion: surprise

(She was in a car, and she was in a bad mood.)

But your example is fine!

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Hi Yoong Liat,

I think by is need here. It sounds more natural to say 'Do you come/go to school by bus or by car?' .

Best wishes,

 Kooyeen's reply was promoted to an answer.
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Thanks, Kooyeen and CJ, for your replies.