A. is wrong because (could'nt) is not the proper way to write the contraction for 'could not' it should be 'couldn't', also there is no need for the apostrophe after the word, yours it already possessive without the apostrophe.
B. is wrong because 'couldnt' is not the correct way to write the contraction for 'could not', it's couldn't.
C. She couldn't remember which suitcase was your's. is wrong because you don't need the apostrophe in your's because it possessive is, yours.
The sentence that contains no errors is D. She couldn't remember which suitcase was yours.
Hope this helps. :)
