Identify the segment in the sentence which contains a grammatical error.
One must not boast about their success.

Correct option is A

Option (a) contains a grammatical error.
Explanation:
The pronoun “one” is singular and formal. It must be followed by “one’s”, not “their”, to maintain consistency.
Correct sentence: One must not boast about one’s success.
Grammatical Rule Used:
Pronoun consistency:
If the subject is “one”, any possessive or reflexive pronoun must also use “one” – not he, their, his, etc.
Example:
· Incorrect: One should improve his habits.
· Correct: One should improve one’s habits.
Information Booster:
In formal writing, mixing pronouns like "one" and "their" is grammatically incorrect.