Correct Answer: The contract must be signed by you before you start working.

Solution : The fourth option is the correct answer.

The transformation involves reordering elements to position the object (the contract) as the new subject, using the appropriate form of the verb to be based on the tense of the original sentence ( must be) for the present tense( must ), changing the verb sign to its past participle form (signed) , and including the original subject (you) with by to denote the doer of the action.

The structure of the passive voice sentence is, " O bject + modal verb + be+ past participle of the main verb+ agent of action + adverbial phrase."

Therefore, the correct answer is, " The contract must be signed by you before you start working".