Select the most appropriate option to substitute the highlighted segment in the given sentence.
If a person tests positive for Covid-19, he have to isolate himself immediately so that he does not infect others.

Correct option is C

The correct substitution of the highlighted words is (c) he has to isolate himself.
Explanation:
The subject “he” is third-person singular, so the verb must agree in form.
· “have to” is used with I/you/we/they
· “has to” is used with he/she/it
Thus, the grammatically correct form is:
→ he has to isolate himself
Grammatical Rule Used:
Subject-Verb Agreement Rule: The verb must agree in number and person with the subject.
· He has to leave now.
· They have to leave now.
Why Other Options Are Incorrect:
· (a) “must have to isolate” – Redundant; "must" and "have to" both express obligation.
· (b) “he isolated himself” – Incorrect tense; does not fit conditional structure.
· (d) No substitution required – Wrong because “he have” is grammatically incorrect.
Corrected Sentence:
If a person tests positive for Covid-19, he has to isolate himself immediately so that he does not infect others.
This correction ensures proper verb agreement and clarity.