What words are the "be" linking verbs
a. before, behind, below
b. am, is, are, was
c. waste, for, and, nor
d. appear, grow, stay, turn

I think the question can be rephrased like this: which of the following forms are forms of "to be", which is a linking verb.

and the answer is B:
all in present:
am - first person singular
is- third person singular
are - plural
and the forth form:
was - simple past