Answer:

Step-by-step explanation:
First, remember what an adjective is. An adjective describes a noun.
Let's examine the sentence.
The slithery serpent slithered
.
Slithery describes the serpent.
Serpent is the noun (person, place, thing, or idea).
Slithered is the verb or action.
is the adverb. It describes the verb. (How did the serpent slither? It slithered silently).
Since slithery describes the noun, it is the adjective.