Final answer:
The correct word to complete the sentence is 'vain,' which implies actions were taken honorably but without success.
Step-by-step explanation:
The phrase in question, "For naught did I in _____, but all in honor," requires a word that fits the context of actions done without resulting in success or value but carried out honorably. The correct word to fill in the blank is c. vain. The complete sentence would therefore read: "For naught did I in vain, but all in honor." This means that despite the actions not yielding the intended results, they were performed with honorable intentions.