How to say "this?" interjection in chinese? For example of a case:

(I was in home walking holding a jar of chocolate candy, I was gonna go to upstairs)
Mom: (She saw me and she called me from behind) "Where did you buy that?"
Me: (I was a bit startled, I turned back and showing up the jar) What?! This? &lt;== I need chinese translation of the bold text.

How about "什么？！这个？” ?
Yes, 什么？ 这个？ is correct.

You could also say something like:

你说的是这个？ (are you talking about this?) to clarify that what they're talking about is in fact the candy.