'A' and 'an' are non-definite articles. They modify nouns that are not specific: their identity is not made known.
'The' is a definite article that modifies a specific noun (its identity is made known).
Option C: In the given sentence, the noun 'door' is made specific by the words 'behind you'. So, the speaker is clarifying which particular door, the one behind 'you'. In this case, we use 'the'.
Hence option C is correct.
Options A and B: As 'a' and 'an' refer to non-specific things, we can't use either over here.
Option D: Since we need an article 'the' for a specific noun, option D is incorrect.