The national anthem ______________ at the super bowl by Whitney Houston.

The phrase "was sung" is correct because it employs the passive voice, which is appropriate in this context. In passive constructions, the subject of the sentence receives the action rather than performing it. Here, the national anthem is the subject, and the action of singing is performed by Whitney Houston.

Using "was sung" accurately indicates that the anthem was sung at the Super Bowl by someone else (Whitney Houston), fitting the structure of the sentence. The past tense form "was sung" also aligns with the time frame of the event, as the Super Bowl performance is a completed action in the past.

Other options do not maintain the required structure or tense. "Was singing" would incorrectly suggest an ongoing action rather than a completed one, "had sung" suggests a different tense that would imply the action occurred before another past action, and "sung" lacks the necessary auxiliary verb to complete the passive voice construction.