The correct active voice of the given sentence is The young students in the annual event were singing the national anthem very enthusiastically.

The passive voice of the sentence is The national anthem was being sung very enthusiastically by young students in the annual event. This sentence is in the present progressive tense, which is used to describe an action that is happening now or that is happening repeatedly. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "the national anthem" and the main verb is "sing." The past participle of "sing" is "sung." So, the passive form of the sentence is The national anthem was being sung very enthusiastically by young students in the annual event.

The active voice of the sentence is formed by putting the subject of the passive sentence before the verb and removing the verb "to be." In this case, the subject of the passive sentence is "young students" and the main verb is "sing." So, the active form of the sentence is The young students in the annual event were singing the national anthem very enthusiastically.

The other options are incorrect because:

• The national anthem was sung very enthusiastically for young students in the annual event. is in the passive voice, but it does not specify who was singing the national anthem.
• The young students in the annual event had been singing the national anthem very enthusiastically. is in the past perfect tense, while the original sentence is in the present progressive tense.
• The national anthem sang the young students very enthusiastically. is grammatically incorrect. The national anthem is not a person, so it cannot sing.