Hello there!
The correct answer should be option A) tienda, which is to say store if you were using it in the Spanish language. The other options wouldn't make sense in this case, because the sentence translated means, Estela buys food at the blank. Seeing how you wouldn't buy food at the B) toystore, c)library, or d) commissary, that leaves the only correct option being A)store.
I hope this helps!