IMO Answer is option (C)
As a citizen of any country, one should acknowledge that one is an integral part of a larger community and exercise their freedom to vote, which, often misunderstood and regarded as uninfluential, forms the foundation of a democratic society.
A. one is an integral part of a larger community and exercise
their freedom to vote, which, often misunderstood and regarded as uninfluential, forms: Singular pronoun required
B. one is an integral part of a larger community and exercise one's freedom to vote, which,
often misunderstood and regarded as not being at all influential, forms:Too wordy
C. one is an integral part of a larger community and exercise the freedom to vote, which, often misunderstood and regarded as uninfluential, forms: Parallel structure.
D. he is an integral part of a larger community and exercise his freedom to vote, which, often misunderstood and regarded as uninfluential, forms
E. one is an integral part of a larger community and exercise one's freedom to
vote, which, often misunderstood and regarded as uninfluential,
form:Needs singular verb