Which are always linking verbs

A. become, remain, seem

B. waste, walk, wonder

C. been, because, yet,

D. am, is, are, was

The correct answer is D.

in C we have some non-verbs, such as "yet"

in A and B we have some verbs that don't have to be linking: for example A. "everyone left, she remained" and B: "she walked" (in B all the verbs are not linking)

in D we only have forms of "to be", which is linking!