Choice C is the best answer. This question tests punctuation with transitional phrases.

1. Context clues: Two independent clauses connected by "as a result."
2. Meaning: Use semicolon before a transitional phrase and comma after it.
3. Verify: "...worked tirelessly throughout the weekend; as a result, they met the deadline" is correct.

💡 Strategy: Use semicolon + transitional phrase + comma when joining independent clauses.

Choice A is incorrect because a comma before "as a result" creates a comma splice. Choice B is incorrect because it lacks the semicolon before the transition. Choice D is incorrect because a colon doesn't precede transitional phrases.