Which of the following words is an adverb?

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.

Source: Grammarly