Select the correct passive form of the given sentence.

Swallows build new nests every year.

New nests are built by swallows every year.

The correct passive form of the given sentence is New nests are built by swallows every year.

The active voice of the sentence is Swallows build new nests every year. This sentence is in the simple present tense, which is used to describe an action that happens regularly or always happens. 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 "swallows" and the main verb is "build." The past participle of "build" is "built." So, the passive form of the sentence is New nests are built by swallows every year.

The other options are incorrect because:

• New nests were built by swallows every year. is in the simple past tense, while the original sentence is in the present simple tense.
• New nests were being built by swallows every year. is in the present progressive tense, which is used to describe an action that is happening now but is not finished. The action of building new nests is something that happens regularly, so the present progressive tense is not appropriate.
• New nests is being built by swallows every year. is grammatically incorrect. The subject of the sentence should be plural, since "new nests" is plural.