Answer: C) was.
Explanation: a verb tense is a clue to help us figure out when the action in a sentence is happening. There are three tenses: past, present and future. To write a logical sentence, the verb tense of all of the verbs should be the same. In the given sentence the verb "hung" is in past tense, so the correct verb to fill in the blank is "was" (also in past tense). So the complete sentence would be: "The forgery was so exquisite that it hung in the museum for the past two ecades without discovery."