The correct answer is actually D), not A.
Although these are independent clauses indeed, the punctuation in A,B, and C is not correct.
Before 'however' there needs to be a semicolon (;). And after however, you have to put a comma. Therefore D is the correct answer.
After a comma there needs to be a pause in reading. Does A sound correct to you if you make a pause before 'however' and not after it? Of course not.
The theme would be a tragic mistake made by the main character. His tragic mistake was his hubris, since he believed he was too strong and that he could do it on his own.
The answer is D. A verb that starts with "to" is in an infinitive verb form. It should be noted that a verb in this form does not function as a verb instead it functions as another part of speech. Infinitives can function as adverbs, adjectives or as nouns.
