The subject of the Active Voice is changed into the object of the Passive Voice. Also, the object of the Active Voice is changed into the subject of the Passive Voice.
The usage of a "to be" form of verb is necessary while changing the voice of the sentence from Active to Passive.
The question in Active voice is in the imperative form. An imperative sentence is used to express order, request, suggestion, etc. They begin with a principal verb. Here, the imperative sentence in Active voice is an order.
The format in Active voice is: V1 + Object.
Thus, the format in Passive voice will be: Let + Object of the active voice (Subjective case) + be + V3.
For example:
Start the match. (Active)
Let the match be started. (Passive)
The options except (a) don't follow the correct grammatical format of the Passive Voice.
Therefore, the correct answer is option (a): Let the access be denied.