The revision that corrects the sentence is D) Heavy, black clouds rushing in from the horizon mean a storm is coming.
The sentence is incorrect because it is incomplete. "Heavy, black clouds rushing in from the horizon" is only the subject of the sentence; therefore, it must be followed by a verb. "Rushing in from the horizon" is not the predicate of the sentence but a participial phrase working as an adjective modifying the noun "clouds". Therefore, option D) is correct because it is the only one that provides a predicate for the incomplete sentence ("mean a storm is coming"). On the other hand, option A), B) and C) leave the sentence incomplete and add another sentence (in the case of A. and B.) or a subordinate clause instead (introduced by "when" in C.).
