Choice C is the best answer. This question tests punctuation with conjunctive adverbs.

1. Context clues: Two independent clauses are connected by "nevertheless."
2. Meaning: Use semicolon before and comma after a conjunctive adverb joining independent clauses.
3. Verify: "...predicted heavy rainfall; nevertheless, the outdoor concert proceeded..." is correctly punctuated.

💡 Strategy: When using conjunctive adverbs (however, nevertheless, therefore) between independent clauses, use semicolon + adverb + comma.

Choice A is incorrect because a comma before "nevertheless" creates a comma splice. Choice B is incorrect because it lacks the semicolon before "nevertheless." Choice D is incorrect because a colon doesn't precede conjunctive adverbs.