Answer:
The correcto sentence is C.
Step-by-step explanation:
Lets review grammar. In the present tense when ever a sentence has (he, she it) we add and "s" to the verb. In this case " The girl who works (is in the present tense).
Other important rule in this sentence is that when conjugating the verb to be (he, she, it) we use "is" for the verb to be in the present tense.
The girl who works at the coffe shop is always friendly.