A sentence has been given in active voice: "The dog killed the cat." Choose the option that correctly expresses this idea in passive voice.

Introduction / Context:
This question checks your understanding of active and passive voice in English grammar. The original sentence "The dog killed the cat" is in active voice, where the subject "the dog" performs the action on the object "the cat". In the passive version, the object of the action becomes the grammatical subject, and the focus shifts from the doer to the receiver of the action. Mastery of voice transformation is important in formal writing, report writing and many competitive examinations, so it is essential to know the correct structure of the passive form for different tenses.

Given Data / Assumptions:

• Active voice sentence: "The dog killed the cat".
• Verb "killed" is in the simple past tense.
• Subject: "the dog".
• Object: "the cat".
• We need a passive voice sentence that preserves both the tense and meaning.

Concept / Approach:
To convert a simple past active sentence into passive voice, the general structure is: object of the active sentence + was or were + past participle of the main verb + by + original subject. The simple past form "killed" remains as the past participle "killed" in the passive. The object "the cat" moves to the subject position, and the subject "the dog" becomes the agent introduced by "by". We must also ensure correct verb agreement and avoid unnecessary change in meaning. Any answer that changes the tense, omits the agent without reason, or produces awkward grammar should be rejected.

Step-by-Step Solution:
Step 1: Identify the subject, verb and object in the active sentence: subject "the dog", verb "killed", object "the cat". Step 2: Move the object "the cat" to the subject position in the passive sentence. Step 3: Use the appropriate form of the verb "to be" for simple past. Since "cat" is singular, we use "was". Step 4: Add the past participle "killed" and then introduce the original subject with "by", giving "The cat was killed by the dog".

Verification / Alternative check:
Check the tense and completeness of the new sentence. "The cat was killed by the dog" is in simple past passive, as required. The meaning is unchanged: the dog caused the death of the cat, and the sentence remains grammatically correct. Now compare with other options. "The cat is killed by the dog" uses present tense "is", which does not match the original simple past tense. "The cat got kill by the dog" is grammatically incorrect because "got" should be followed by "killed", not "kill". "The dog was responsible for killing the cat" changes the structure and is not a direct passive form of the sentence, though it expresses a similar meaning. "The cat had been killed by the dog" changes the tense to past perfect, which is not required here. This confirmation shows that only one option fully satisfies the passive transformation rule.

Why Other Options Are Wrong:
Option "The cat is killed by the dog" shifts the action to the present, breaking the tense agreement. Option "The cat got kill by the dog" is ungrammatical because it should read "got killed". Option "The dog was responsible for killing the cat" adds the phrase "was responsible for" and does not follow the strict active to passive conversion pattern. Option "The cat had been killed by the dog" uses the past perfect passive, which suggests a different time relation and therefore does not accurately mirror the original simple past statement.

Common Pitfalls:
Learners often forget to keep the tense consistent when converting from active to passive. Another frequent error is using the wrong form of the verb "to be" or the wrong participle. Some students also assume that any sentence that keeps the same general meaning is acceptable, even if the grammatical pattern has changed. In examinations, however, the required answer is usually the strict structural transformation. Always check the original tense, ensure correct "was" or "were", and keep the agent introduced by "by" when it is present in the active sentence.

Final Answer:
The correct passive voice form of "The dog killed the cat" is The cat was killed by the dog.