Answer :
The correct answer is B, which is indeed a run-on sentence. There are two independent sentences. A period should be placed after the word continues, with a new sentence starting with the weather. The semi-colon in example D is correct, making this a solid (not a run-on) sentence.
The correct answer is B. The game will be called if the storm continues, the weather is dangerous.
A run-on sentence means that two sentences are connected in a grammatically wrong way, usually by writing a comma between them. This is sometimes wrong, as they need to be connected using a conjunction as well. So, in order to fix sentence B, you'd need to write the conjunction and if before the weather is dangerous.