In the following sentence, choose the option that best replaces the bracketed part to make the sentence grammatically correct. If no improvement is required, select the option labelled no improvement. Someone must (being) killed.

Introduction / Context:
This question tests knowledge of how modal verbs combine with perfect aspect to describe a strong logical conclusion about a past event. In English grammar, forms like must have been, must be, and must be being are often confused by learners, especially when used with passive voice structures such as killed, arrested, or punished. The sentence here suggests that something very serious has happened in the past, and we have evidence that makes the speaker almost sure someone lost life. Therefore, the verb phrase in brackets needs to be corrected so that it clearly expresses a deduction about a completed past action.

Given Data / Assumptions:
The information in the sentence is limited, but we can still note the key points and rely on general grammar rules about modals and passives.

The main clause is Someone must (being) killed.
The word must is a modal verb expressing strong probability or near certainty.
The verb killed is a past participle used in a passive structure.
The bracketed word being is ungrammatical in this pattern.

Concept / Approach:
When we use must to talk about a logical conclusion about a past event in passive voice, the normal structure is must + have been + past participle. The form must have been killed means that, based on evidence, the speaker concludes that a killing has definitely happened in the past. The structure must be killed would describe necessity in the present or future, not a deduction about a completed event. The structure must being killed is simply wrong in standard English. Therefore, the bracketed part needs to become have been so that the full verb phrase reads must have been killed.

Step-by-Step Solution:

Verification / Alternative check:
We can verify the correctness by comparing with similar standard sentences, such as There is blood on the floor, someone must have been hurt or The lights are off and the door is locked, they must have gone home. In each of these, must have + past participle describes a logical conclusion about a past action. Applying the same rule, must have been killed is the only correct and natural pattern in formal English. Reading the full sentence aloud confirms that Someone must have been killed sounds natural, while the other options feel incomplete or incorrect.

Why Other Options Are Wrong:
Option has been gives Someone must has been killed, which is ungrammatical because must cannot be followed by a present perfect form with has. Option been would give Someone must been killed, which is also incorrect because a helping verb like have or be is missing. Option to be would give Someone must to be killed, which is not correct either, because modals take the bare infinitive be rather than to be, and it would also suggest future necessity, not a past conclusion. Option no improvement keeps being, resulting in Someone must being killed, which is clearly wrong in standard English grammar.

Common Pitfalls:
A common error is to ignore the time reference and simply use must be with any past participle, such as Someone must be killed, which actually describes something that should happen in the present or future rather than something that already happened. Another frequent mistake is mixing must with has or had directly, for example must has been, which is never correct in English. Learners also sometimes confuse continuous passive forms like is being killed with perfect passive forms like has been killed, but here the context clearly points to a completed past action. Remember that must have + past participle is the standard pattern for strong logical conclusions about the past.

Final Answer:
The correct improvement of the bracketed part is have been, so the sentence should read Someone must have been killed.