Correct Option : B

Explanation:

While transforming imperative sentences from active to passive voice we insert either let if the verb is transitive or we introduce various ideas like asked, requested, begged, ordered, commanded, told, advised and so on depending on the situation.

So the correct transformation would be.

You are urged / insisted / heartily requested / advised to get prepared for the exam.

Alternatively, following sentences are also valid.

You are advised to be prepared for the exam.

Be prepared for the exam. 

Note: Helping verb ‘Be’ is used before the main verb ‘prepare’ which makes it more emphatic or urging.