The forgery so exquisite that it hung in the museum for the past two decades without discovery.

which verb or verb phrase is in the correct tense to complete the sentence?

should be
is being
was
will be

B) Was.

Explanation: the tense of a verb shows the time when an action or condition occurred. There are three basic tenses: past, present, and future. They show whether a simple action or condition occurred, occurs, or will occur in the past, present, or future. In the given sentence the verbs should be in past tense (we can see that because of the use of the verb "hung"), so the correct answer is "was" which is also in past tense.

c. a group of tribes that formed a confederacy