In the following question, out of the four alternatives, select the alternative which will improve the bracketed part of the sentence. In case no improvement is needed, select "no improvement". He must not (to be left) like this.

Introduction / Context:
This grammar question checks correct use of modal auxiliary verbs with passive voice. The sentence is He must not (to be left) like this. The phrase in brackets is incorrect, and we must choose the correct form from the options. Modal verbs such as must, can, may, and should are always followed by the base form of the main verb, not by a to infinitive in this structure. The sentence also uses a passive construction, because something must not be done to him.

Given Data / Assumptions:

• The subject is He.
• The modal is must, showing strong necessity or prohibition.
• The sentence has a passive idea: he must not be left like this, meaning people should not leave him in such a condition.
• The bracketed phrase is to be left, which needs correction.
• Options include being left, been left, be left, no improvement, and to have left.

Concept / Approach:
The rule is that after a modal auxiliary such as must, the next word is the base form of a verb, not to plus verb, not ing form, and not past participle alone. For passive voice, the base form of be is used, followed by the past participle of the main verb. Therefore, must not be left is the correct pattern: must (modal) plus not (negation) plus be (base form) plus left (past participle). Recognising this structure allows us to identify be left as the only grammatically correct replacement for the bracketed part.

Step-by-Step Solution:
Step 1: Recall that the structure after must is must plus base verb. Step 2: To form the passive voice of leave we use be left. Step 3: Therefore the correct string after must not should be be left, giving He must not be left like this. Step 4: Check Option C, be left. When inserted, it produces a sentence that is grammatically correct and natural. Step 5: Option A, being left, would give He must not being left like this, which is ungrammatical because being is not the base form. Step 6: Option B, been left, equally fails because been cannot come directly after must without a form of have before it. Step 7: Option D, no improvement, would keep to be left, which is not correct after must in this kind of sentence. Step 8: Option E, to have left, changes the meaning and tense and also breaks the passive structure. Step 9: Therefore, be left is the correct improvement.

Verification / Alternative check:
Consider similar sentences. We say He must not be disturbed, not He must not to be disturbed. We also say They must not be allowed to enter, not They must not to be allowed to enter. In every case, the modal must is followed directly by be and a past participle. By analogy, He must not be left like this is the correct pattern. This comparison with common structures confirms that Option C is correct.

Why Other Options Are Wrong:
Being left suggests a continuous form and would require an auxiliary like is or was before it, as in He is being left, which is not the structure here. Been left is part of a perfect passive form and would need have before it, leading to He must have been left, which changes the tense and not the prohibition. No improvement keeps the ungrammatical to be left directly after must not. To have left is an active perfect infinitive and would describe his action of leaving, not the passive idea that others might leave him.

Common Pitfalls:
Students sometimes think that any form of be is acceptable after a modal, but the rule requires the base form, not participles. Another pitfall is thinking that the original phrase must be correct because it looks similar to other structures like have to be left. However, must and have to behave differently, and must not to be left is not standard usage. Careful attention to typical patterns such as must be done or must not be done greatly improves accuracy in such questions.

Final Answer:
The correct improvement is be left, so the sentence becomes He must not be left like this.