So or although?

The answer depends on context. Specifically, was 'she' not hungry before or after she ate the food?

If before, the answer would be B. Otherwise, it would be A.

However, D would also be grammatically correct (but, it does sound unnatural when you say it in your mind).

BoldBen wrote in a comment:

I can see cases where A, B and D would all be appropriate, although C seems very unlikely. It would depend on the context, if the questioner didn't provide any context I would find it difficult to choose the one they wanted.

Lawrence wrote in a comment:

Syntactically and grammatically, all options (A to D) fit. Semantically, C seems odd. Without context, D seems disjointed. For B, the exam setter might have preferred had eaten and perhaps a comma after food (though that doesn't invalidate B). None of these issues are problems for A.

oerkelens wrote in a comment:

Really A, B and D are all suitable, so if that was indeed the question, all three of those should be deemed correct. Question writers are a special breed of devil, it seems.

Your sentence says that She ate a lot of food even though she wast't hungry. "Even though" is the synonym of "although", so you will have to go with A. All the other options are incorrect and can not fit in your sentence.