Concept:The pronoun after a singular indefinite subject like "every person" must be singular to maintain subject-pronoun agreement.
Explanation:The subject "Every person" is singular.
Singular subjects require singular verbs and singular pronouns.
In part (c), the pronoun "their" is plural and does not agree with the singular subject.
The correct singular pronoun to use is "his" (or "her", but traditionally "his" is used in such general statements).
Therefore, part (c) contains an error.
The correct sentence is: "Every person who believes in principles must stand up to fight for his convictions."
Parts (a) and (b) are grammatically correct.
Hence, the error is in part (c) only.
Answer:(c)