Choose the option that is the passive form of the sentence.

They found her guilty of theft.

She was found guilty of theft.

The correct option that is the passive form of the sentence is She was found guilty of theft.

The other options are incorrect because:

• She is find guilty of theft by them is grammatically incorrect. The verb "to find" is followed by a direct object, but the phrase "is find" is not a direct object.
• She found them guilty of theft is in the active voice.
• She had been find guilty of theft is in the past perfect tense, while the original sentence is in the simple past tense.