In the sentence "One should exercise their right to vote", which pronoun best replaces "their" to maintain correct formal usage with the indefinite subject "One"?

Introduction / Context:
This sentence improvement question tests your knowledge of formal pronoun agreement in English, especially with the indefinite subject one. The sentence One should exercise their right to vote mixes one with their. In strict formal usage, there is a standard pattern that should be followed. You must choose the option that best matches this pattern and keeps the sentence grammatically and stylistically correct for exam standards.

Given Data / Assumptions:

• Sentence: One should exercise their right to vote.
• Subject: One, used in a general sense to mean any person in general.
• Pronoun replacing their must refer back to one in a consistent way.
• Options: his, our, one with possessive ending, and No improvement.

Concept / Approach:
In traditional and formal English, when the subject is one, the possessive pronoun that follows is written as one with a possessive ending, spoken as ones with an apostrophe. The pattern is one should do one duty or one should exercise one right to vote. Using their is increasingly common in modern informal English to avoid gender bias, but most competitive exams still expect the classical one plus possessive form. Therefore, replacing their with the possessive form of one gives the most formally correct sentence.

Step-by-Step Solution:
Step 1: Note that the subject is one, a singular indefinite pronoun used for general statements.Step 2: Recall the standard pairing in formal grammar: one should take care of one responsibilities, using the possessive form of one.Step 3: Test each option: his would give One should exercise his right to vote, which introduces a specific male reference and does not perfectly match the neutral one.Step 4: Our would give One should exercise our right to vote, wrongly shifting from singular general one to plural inclusive our.Step 5: The possessive form of one maintains consistency: One should exercise that general person right to vote, and is the choice preferred in formal exam writing.

Verification / Alternative check:
Consider other examples: One must do one duty for the nation. Here, the repetition of one and its possessive form is the classic pattern taught in grammar books. If we say One must do their duty, many modern speakers accept this, but formal exam keys usually mark it as less precise. Similarly, One should guard one health is considered formally correct. In this exam context, the safest answer is the option that gives the possessive form of one, not their, his, or our.

Why Other Options Are Wrong:
His is grammatically possible, but it introduces a gender specific pronoun for a gender neutral subject and is therefore not ideal in modern exam settings that aim for gender neutrality. Our is clearly incorrect because it mixes a singular subject one with a first person plural possessive. No improvement cannot be chosen because the sentence is constructed to highlight the inconsistency between one and their, which the exam setter wants you to correct.

Common Pitfalls:
Learners often struggle with balancing formal rules and modern usage. In everyday conversation, many native speakers now say Everyone should do their best or One should exercise their right to vote, so the use of their feels natural. However, in competitive exams that follow classical grammar guides, one is paired with its own possessive form. The best strategy is to learn the conservative rule for exams while also being aware of real life usage.

Final Answer:
The pronoun that best replaces their while keeping consistent formal usage with one is the possessive form written as one with an apostrophe and s, so the correct option is one's, and option C is correct.