Explanation
Correct answer: C.
Choice C is the best answer. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are both independent clauses, so using a semicolon to separate them is grammatically correct.
Why the other choices are wrong
Choice A
Choice A is incorrect. This choice creates a run-on sentence error. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are both independent clauses, so a comma is not enough to separate them.
Choice B
Choice B is incorrect. This choice creates a run-on sentence error. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are both independent clauses, so they need to be separated with specific punctuation (a period, a semi-colon, a colon, a dash, or a comma + a coordinating conjunction).
Choice D
Choice D is incorrect. This choice creates a run-on sentence error. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are both independent clauses, so the word “and” by itself is not enough to separate them. There would need to be a comma before “and” for this choice to work.