Which of the following options represents the correct transformation of the given sentence into the passive voice?

Do they not post parcels to the clients each afternoon?

The correct option is Aren't parcels posted by them to the clients each afternoon?

To transform the given active voice sentence into the passive voice, follow these step-by-step grammatical rules:

1. Identify the components of the active sentence:
- Active Sentence: "Do they not post parcels to the clients each afternoon?"
- Tense: Simple Present Tense (indicated by the auxiliary verb "Do")
- Subject: "they"
- Verb: "post"
- Direct Object: "parcels"

2. Apply Passive Voice conversion rules for Simple Present Negative Interrogative sentences:
- The direct object of the active sentence ("parcels") becomes the subject of the passive sentence.
- Since the new subject ("parcels") is plural, we use the plural form of the auxiliary verb 'to be' for the present tense, which is Are (or negative contracted form Aren't).
- The main verb ("post") must be changed into its past participle form, which is posted.
- The original subject ("they") changes to the objective pronoun them introduced by the preposition "by" (i.e., "by them").
- The remaining prepositional phrase and time modifier ("to the clients each afternoon") remain intact in their position.

3. Formulate the Passive Voice structure:
[Auxiliary Verb (Aren't)] + [New Subject (parcels)] + [Past Participle (posted)] + [Agent (by them)] + [Remaining phrase (to the clients each afternoon)]?

Combining all elements, we get:
"Aren't parcels posted by them to the clients each afternoon?"

Therefore, the sentence correctly transformed into the passive voice is Aren't parcels posted by them to the clients each afternoon?