Answer:
C. For
Step-by-step explanation:
An independent clause, is a clause or a phrase, with a subject and predicate, that can stand on it's own as a complete sentence. If we separate the sentence above, we can see that both parts, can stand as a complete sentence.
Sentence: One should make the most out of today, for one never know what tomorrow will bring.
Independent clause(s): One should make the most out of today. One will never know what tomorrow will bring.
These two sentences make sense on their own, as complete thoughts. The use of the conjunction, (the comma), and the preposition, (for), just combines these thoughts into one sentence.