Yesterday rachel worked slowly because she was so tired.

which words in the sentence function as adverbs?

Adverbs are words that modify adjectives or verbs.
Many adverbs end in -ly, such as kindly, politely, softly, etc.

Yesterday can be either an adverb or a noun. In the way it's used here, it is an adverb.

Rachel is a name, so it is a noun, or proper noun to be precise.

worked is a verb, since it's an action. (to work)

slowly is an adverb, since it describes the verb, worked.

because is a conjunction.

she is a pronoun.

was is a verb.

so is an adverb, it describes the verb, was.

tired is an adjective.

So the answers to this question are Yesterday and so.

Hope this helps :)

It would be if there was a picture !

Dwould be the answer