The correct answer is C. interjection
Interjections are words loaded with emotion - when you are using them, their meaning is quite clear. For example: Ouch! Yikes! Oh no! Boo!, etc. These are not used in formal writing, whereas it is quite difficult to write a normal sentence without the use of conjunctions (and, or, when, etc), prepositions (in, with, under, etc.), or adverbs (quickly, smoothly, etc.).