Do not write the letter. Change into Passive voice
Options
Let not the letter written
Let the letter be not written.
Let the letter not be written.
Let the letter not be wrote.

The option that has the correct passive form of the sentence is:

C. Let the letter not be written.

Explanation:

While the active voice emphasizes the performer of the action, the passive voice emphasizes the receiver. Take a look at the examples below:

Active: I cut the tree.

Passive: The tree was cut by me.

In general, the object of the active voice becomes the subject of the passive voice. What was the subject in the active then becomes the agent in the passive. But, what happens when we have an imperative sentence?

Active: Do no write the letter.

In this case, the structure we need to use to transform the sentence is:

let + the object + not + be + past participle of the main verb

Since the object in the original sentence is "the letter", the sentence becomes:

Passive: Let the letter not be written,