Select the correct passive form of the given sentence.

A child could understand his theory.

His theory could be understood by a child.

The correct passive form of the given sentence is His theory could be understood by a child.

The active voice of the sentence is A child could understand his theory. This sentence is in the simple past tense, which is used to describe an action that happened in the past. 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 "a child" and the main verb is "understand." The past participle of "understand" is "understood." So, the passive form of the sentence is His theory was understood by a child.

However, the sentence also implies that the child could understand the theory, which is why the modal verb "could" is used. The passive form of the sentence with "could" is His theory could be understood by a child.

The other options are incorrect because:

• His theory was understood by a child. is in the simple past tense, while the original sentence is in the present tense.
• His theory can be understood by a child. is in the present tense, but the action of understanding the theory is not something that is happening now.
• His theory is being understood by a child. is in the present progressive tense, which is used to describe an action that is happening now. The action of understanding the theory is not something that is happening now.