Hi Vipul,
Option C does not fit because it disrupts the parallel structure of the sentence and changes the intended meaning.
The correct phrase, "The more people change, the more they remain the same," uses a parallel structure ("The more... the more...") to emphasize a paradoxical idea: even as people or things change, some underlying aspects stay the same.
If we choose option C, the sentence would read: "The more they are different, the more they remain the same." This does not logically follow because if something becomes more different, it wouldn't simultaneously remain the same. The meaning becomes contradictory without the intended paradox. Also, this construction is problematic because the pronoun "they" in the second part of the sentence becomes ambiguous and undefined. There's no clear antecedent for "they," making the sentence confusing.
In contrast, option B clearly establishes "people" as the subject, so when the sentence continues with "the more they remain the same," the pronoun "they" is clearly understood to refer to "people." This maintains clarity and preserves the intended meaning of the sentence.
I hope this helps.