This question tests your understanding of passive voice formation in English grammar, specifically for interrogative sentences (questions) in the past continuous tense.

The original sentence is: "Why was the police checking all the cars?"

This sentence is in the active voice. Let's break it down to convert it to the passive voice.

Step 1: Identify the Sentence Components

• Subject (the doer of the action): the police
• Verb: was checking (past continuous tense)
• Object (the receiver of the action): all the cars
• Question Word: Why

Step 2: Rules for Passive Voice in Past Continuous Tense

The structure for a passive voice sentence in the past continuous tense is:

Object + was/were + being + past participle (V3) + by + subject

Since this is a question starting with "Why", we must also invert the structure.

Step 3: Apply the Rules

Let's apply the conversion:

1. The object (all the cars) becomes the new subject.
2. Because "all the cars" is plural, we must use the plural verb were.
3. The verb changes to were + being + checked (past participle of 'check').
4. The original subject (the police) is introduced with by.
5. The question word Why is placed at the beginning.

This gives us the correct structure: Why + were + all the cars + being + checked + by the police?

Step 4: Evaluate the Options

Now, let's see which option matches our correct structure.

• Option 1: "All the cars were police checked."
  This is a statement, not a question. It also incorrectly uses "police" as an adjective. Incorrect.
• Option 2: "Why was police checked all the cars?"
  This is grammatically incoherent. The word order and verb agreement are wrong. Incorrect.
• Option 3: "Police was checking all the cars."
  This is a statement in the active voice, not a question in the passive voice. Incorrect.
• Option 4: "Why were all the cars being checked by the police?"
  This perfectly follows all the rules we outlined:
  • Starts with the question word "Why".
  • Uses the correct plural verb "were" to agree with the plural subject "all the cars".
  • Uses the correct passive structure "were being checked".
  • Includes the agent "by the police".
  This is the correct answer.

Final Answer

The most appropriate passive form is: Why were all the cars being checked by the police?

Related Topic: Active and Passive Voice

Theory: Voice is a form of a verb that shows whether the subject performs the action (active voice) or receives the action (passive voice). The passive voice is used when we want to emphasize the action or the object of a sentence rather than the subject. The structure always involves a form of the verb "to be" (e.g., is, was, were, been) plus the past participle of the main verb.

Key Formulae (Rules)

General Structure for Passive Voice:
Object + form of 'to be' + Past Participle (V3) + [by + subject]

For Past Continuous Tense (Passive):
Object + was/were + being + Past Participle (V3) + [by + subject]

For Interrogatives (Questions):
The question word (Why, What, When, etc.) is placed at the beginning, followed by the inverted passive structure (e.g., Were + object + being + V3...?).