Concept: Conjunctive adverbs like 'however' at the beginning of a sentence must be followed by a comma to mark the transition.Rule / Clue: The word 'however' here introduces a contrast, so it is an introductory element requiring a comma.Explanation: In option C, the comma after 'However' correctly separates it from the independent clause 'he tried again'. Option A wrongly puts a comma between subject and verb, breaking grammar. Option B places a comma after the verb, which is incorrect. Option D omits the necessary comma, making the sentence run-on. Only option C follows standard punctuation rules for transitional adverbs.Answer: Option C. However, he tried again.