In choice (A), the phrase preceding the semicolon is ungrammatical. A semicolon always separates independent clauses, but, as written, the clause before the sentence is not an independent clause, because it starts with the subordinating conjunction "while." On these grounds, we can eliminate choice (A) and also choice (B).
Choice (C) fixes the problem of choice (A) and (B) by not using any subordinating conjunction. The meaning is clear and the grammar is correct.
In choice (D), the pronoun "they" has no clear reference; there is no logical plural noun for it to refer to, so we eliminate (D). We can eliminate choice (E) on the same grounds.
The correct answer is (C)