If you ask someone something, you say something to them in the form of a question because you want to know the answer.

Q1) The reason 'the' is used there is the speaker of the sentence thinks of the specific form of a question? For example, in his head he may think of a question starting with 'Does it / Is this/ Are you etc.?'

Q2) If the speaker uses a instead of 'the,' he does not think of any specific form of a question?
Q1) The reason 'the' is used there is the speaker of the sentence thinks of the specific form of a question? For example, in his head he may think of a question starting with 'Does it / Is this/ Are you etc.?'-- Vaguely, yes. For the purposes of this utterance, questions have one form and statements have another form.

Q2) If the speaker uses a instead of 'the,' he does not think of any specific form of a question?-- S/he wouldn't use 'a' there.
... say something ... in the form of a question ...

in the form of a question = in [the / that] form that [a / any / every] question (always) has = in question form

Here the speaker is thinking of "question form" (or "the form that a question has") as one specifically defined thing. From the speaker's point of view, there is only one thing that qualifies as "question form", and he believes that the listener (or reader) knows what that is. He believes that by saying "form of question" he has specified exactly what he's talking about, that he will not be asked "Which form of a question are you talking about?" So he uses "the".

Compare:

Could alien life exist in the form of DNA dust? (in that form that DNA dust has)

The British Museum has a pottery lamp in the form of a ship. (in that form that a ship has; in ship form)

News article:

Reagan in the Form of a Cake. (in that form that a cake has; in cake form)

This cake was on display at last night's CPAC dinner, a celebration of the centennial of Ronald Reagan's birth.

Note that in these cases, the speaker does not expect to be asked "Which?" Which form of DNA dust? Which form of ship? Which form of cake? The reason is that he believes that there is only one thing that can be meant by 'DNA-dust form', 'ship form', or 'cake form'.

________________

Contrast:

a form of = one of the ([several / many]) forms of

You are likely engaging in a form of market timing. (in one of the forms of market timing)

A member may practice public accounting only in a form of organization permitted by law. (in one of the forms of organization permitted by law)

This gene, mutated in a form of deafness, is expressed in the central auditory pathway. (in one of the forms of deafness)

Note that in all the cases of "a form of", the speaker realizes that he may legitimately be asked "Which?" - Which form of market timing? Which form of organization? Which form of deafness? - because there may be different forms of market timing, organization, or deafness.

CJ
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Thank you very much Micauber and CJ

I just accepted Micauber's answer without reasoning; the speaker wouldn't say a type of question~~.

Your answer with easily patterned examples helped me understand why the speaker woundn't say a type of ~~ in the context.

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