Concept:Comparative adjectives compare two things. They often use 'than' and end in '-er' or use 'more'.Rule / Clue:The word 'than' in the sentence signals a comparative form. Only the comparative degree fits here.Explanation:Option A 'might' is the base adjective, not comparative. Option B 'mightiest' is superlative (comparing three or more). Option C 'mightier' is the comparative form of 'might', correct for comparing pen and sword. Option D is wrong because 'mightier' is valid.Answer:C. mightier