The correct answer is (B).
Getting to the Answer: When the underlined portion contains punctuation, check to make sure that it is used correctly. In this case, a comma separates two independent clauses, which creates a run-on. The correct answer choice will properly separate the two independent clauses.
(A) Incorrect: a comma is not used to separate two independent clauses
(B) Correct: a period is used to separate two independent clauses
(C) Incorrect: a comma is not used to separate two independent clauses
(D) Incorrect: a comma is not used to separate two independent clauses; the second clause is still independent, as this choice simply changes the subject of the clause from "forecast" to "that"