Which part of speech is the underlined word?

that's what a fast is—a period of time during which you do not eat.(fast being the underlined word)

a.
noun

b.
adverb

c.
conjunction

d.
preposition

in the secret life of walter mitty, the main character leads a dull life, or so he thinks.in all his daydreams he plays the role of some hero or a man who is unbelievably resourceful.whether a pilot, a surgeon   or even a criminal, he is better, so much better than all the rest of the characters in his delusions.he comes up with ideas nobody else could have conceived .when he comes back to reality, whatever he has to deal with seems silly, boring without any motivation to deliver.

in a sound of thunder a hunter,eckles, wants to use a time machine to go back in time to catch a tyrannosaurus rex. by then, 2055 elections were go held and a   new hitler was beaten by a far better candidate, keith.once he and other men have traveled,   he gets cold feet. he in   some way has known   this is not a good idea.he hears this sound , too strong for him to put up with . hopeless, he abandons the project making a mistake - another one as the first is to travel in time-that causes a butterfly effect when he comes back. in the end , changes reality for worse; the anti human anti christ deutscher,   is the president.

free points are committed here! first to answer by writing this question and post it as an answer gets brainliest and everybody gets 30 !