Since the prisoner couldn’t coexist by another convicts, he spent a lot of time in solitary confinement.
Which of the following phrases given below the sentence should replace the phrase printed in bold letters to make the sentence grammatically correct? Choose the best option among the five given alternatives that reflect the correct use of phrase in the context of the grammatically correct sentence. If the sentence is correct as it is, mark “No improvement required” as the answer.

The correct answer is coexist with other convicts, he.

Let's analyze the grammatical structure of the sentence step-by-step to understand why this option is correct:
1. Prepositional Agreement: The verb "coexist" means to exist together at the same time or in the same place. The correct preposition that follows "coexist" when referring to people is "with" (e.g., to coexist with someone), not "by". Therefore, "coexist by" in the original sentence is grammatically incorrect.
2. Determiner and Noun Agreement: The word "other" should be followed by a plural noun ("convicts") in this context to indicate a general group of people, rather than the singular "convict".
3. Modal Verb Rule: The modal auxiliary verb "couldn't" (could not) must be followed by the base form of the verb. Thus, "coexist" is correct, whereas "coexists" or "coexisted" would violate this grammatical rule.
4. Pronoun Agreement: The pronoun referring back to "the prisoner" must be "he" to maintain gender/person agreement, not "it".

Putting it all together, the phrase "coexist with other convicts, he" correctly rectifies the preposition error and maintains proper subject-verb and pronoun-noun agreement, making the complete sentence grammatically correct: "Since the prisoner couldn’t coexist with other convicts, he spent a lot of time in solitary confinement."