C.
This is because an adverb is a word that modifies/describes a verb -he sings loudly-, an adjective -very tall-, another adverb -ended too quickly-, or even a whole sentence -Fortunately, I had brought an umbrella-. Adverbs often end in -ly, but some -such as fast- look exactly the same as their adjective counterparts. So, everything but answer C. is actually an adverb.
Hope this helps! I know the other guy answered but I wanted to anyway.
I also just noticed this was asked in 2016 but oops.