What month is your birthday?

[kadioguy]

[My conversation]

A: What month is your birthday?
B: Well, my birthday is in September.
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Why is there no "in" in the question, while there is one in the answer?

What do you think of them?

a. What month is your birthday?
b. In what month is your birthday?
c. What month is your birthday in?
d. In which month is your birthday?

 

[jutfrank]

Why is there no "in" in the question, while there is one in the answer?

You wrote it so you tell us!

 

[kadioguy]

You wrote it so you tell us!

I had a feeling that it is correct to write that way.

 

[jutfrank]

Well, your feeling was right. It's a natural way of asking this question in casual spoken English even though it doesn't make sense analytically.

And yes, you need in in the answer if you want to make a complete sentence. However, it would be better and easier just to say September as the questioner requires only a one-word answer.