As a response

[99bottles]

As a response/In response, she stuck out her tongue.

Are both choices correct/natural?

 

[Tarheel]

I think both of them are grammatically correct (with the second one preferred). However, more natural, in my opinion, would be:

Then she stuck out her her tongue. (I assume it's in response to something.)

It's hard to say definitely without knowing the context, so it's just a general opinion.

 

[emsr2d2]

I'd use only "In response".

 

[Rover_KE]

Stick to 'in response'. [link]

(Bookmark the Ngram site for future reference.)