Help! A choice.

I would say choice a
What do you think he bought the computer?
if you use the other choices, you will have 2 structures of interrogation in the same question
the first one, what do you think
the second one, "has he bought", or "did he buy", or "had he bought"
and you can't have 2 forms of interrogations in the same question

1. What do you think he bought the computer? ] This suggests he bought something to give to the computer, as a present. Not possible! Though you could say:

1a. What do you think – he bought the computer!

However, this doesn't fit the context.

2. What do you think has he bought the computer? ] This would only be possible if repunctuated: What do you think – has he bought the computer?

3. What do you think did he buy the computer? ] Ditto: 'What do you think – did he buy the computer?'

4. What do you think had he bought the computer? ] Not possible, even if repunctuated.

5. Why do you think he bought the computer for? ] Not possible; change to 'What do you think he bought the computer for?' or 'Why do you think he bought the computer?'

So as the question stands, 2 and 3 are both possible, if the sentence is repunctuated.

MrP

Hello Temico

Yes; in standard English, you would have to say either 'why do you think she came to the school', or 'what do you think she came to the school for'.

(Though no doubt there are dialects where 'why...for?' is permissible!)

MrP

"Why do you think they are going to Bangkok, if not for the girlies?"

or

"What do you think they are going to Bangkok for, if not for the girlies?"

would seem more likely.

(Obviously you've heard your colleagues so I'm not doubting what they've said, but the above two variations are the way a native BrE speaker would phrase the sentence).