Technically the answer to this can be two of the three options, A. and C. (although A. is likely to one your teachers is looking for. I will explain why below)
I have an apple
vs.
I have the apple
It may not seem like it, but these sentences tell us to different things.
When you say a/an this means that there are likely several apples in the vicinity and/or there is nothing particular or unique about the apple you have. In this case the reason you would say "an apple" and not "a apple" is because "apple" starts with a vowel. When a word starts with a vowel you must use "an" instead of "a."
When you say "the apple" it is likely that the apple is unique or has some particular quality. It's like saying that it is a one of a kind apple.
Hope this helped!
~Just a girl in love with Shawn Mendes
