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