The correct option that expresses the given sentence in passive voice is:

☀ Let the flowers not be plucked.

So, the correct passive form is: "Let the flowers not be plucked."

The other options and their explanations:

☀ The flowers should not be pluck.

• This option is close to the passive voice, but it contains a grammatical error. The correct form should be "plucked" (past participle) instead of "pluck" (infinitive) to maintain the passive voice.

☀ You are requested not to plucked the flowers.

• This option is in the active voice and contains a grammatical error. It should be "plucked" (past participle) instead of "plucked" (infinitive).

☀ I am requesting you to pluck the flowers.

• This option is also in the active voice and conveys the opposite meaning of the original sentence. Instead of asking someone not to pluck the flowers, it requests them to pluck the flowers.