Generally when a word ends with 'x' already, we add an 'es' to the word to make it plural. We do not add an apostrophe (the symbol ') as that is used to show possession, not plural. This rules out Option D); and Option B) is incorrect as it is not even a word. This leaves us with the correct answer - Option C) and NOT A) - as there is an exception here. The word 'appendices' is the specific plural for the singular form 'appendix' making this an exceptional case.