In the following question, a sentence in Active voice is given. Select the option that best expresses the same meaning in Passive voice: They lock the door every night.

Introduction / Context:
This question tests your understanding of voice transformation, specifically converting an active voice sentence in the simple present tense into its passive voice equivalent. Such conversions are very common in grammar sections of competitive exams.

Given Data / Assumptions:
The active sentence is They lock the door every night. The subject is they, the verb is lock in simple present tense, and the object is the door. The adverbial phrase every night describes frequency. In passive voice, the object the door becomes the grammatical subject, and the tense and aspect must match the original sentence.

Concept / Approach:
To convert an active simple present sentence of the form subject plus base verb plus object into passive, we use the structure object plus is or are plus past participle plus by plus subject. Here, the door is singular, so we use is locked. The phrase every night should remain at the end of the sentence, and the agent they can be retained as by them, although in real life it is often omitted. The key is to preserve tense and meaning.

Step-by-Step Solution:
First, identify the tense of the active sentence. They lock the door uses the simple present form lock, not a continuous or perfect form.Second, form the passive auxiliary with the correct be verb. Since the door is singular and the tense is simple present, we use is.Third, use the past participle of lock, which is locked.Fourth, bring in the agent phrase by them at the end to show who performs the action, and keep every night as the frequency adverbial.This yields The door is locked every night by them, which matches option D.

Verification / Alternative check:
We can check correctness by converting back to active voice. From The door is locked every night by them, we produce They lock the door every night. The tense, subject, object, and frequency phrase all match the original sentence, confirming that we have made a correct and reversible transformation.

Why Other Options Are Wrong:
Option A uses the past continuous passive was being locked, which changes the tense and aspect to past continuous and misrepresents the habitual present meaning. Option B uses has been locked, which is present perfect passive, again changing the tense and suggesting that the action started in the past and continues to the present. Option C uses had been locked, which is past perfect passive and does not match the simple present habitual sense. These options therefore do not correctly reflect the original tense and meaning.

Common Pitfalls:
Some learners mechanically choose any passive form that seems familiar without paying attention to tense. Others confuse the simple present with the present perfect, especially when adverbs like every night are present. To avoid these mistakes, always identify the tense and aspect of the original verb first, then construct a passive form that uses the same tense and aspect with the appropriate form of be and the past participle.

Final Answer:
The correct passive sentence is The door is locked every night by them.