The sentence is in the past tense, so the verbs "trained," "screamed," and "fought" should also be in the past tense for consistency. "Fights" is in the present tense, which is not appropriate for this context.
Option A is therefore incorrect because it uses "fights," which is present tense.
Option B ("fighting valiantly") uses the present participle form of the verb, which indicates an ongoing action. However, the sentence is describing a completed action, so the past tense form of the verb is needed.
Option D ("to fight valiantly") uses the infinitive form of the verb, which is not appropriate for this sentence because it does not agree in tense with the other verbs in the sentence.
Option C ("fought valiantly") is the correct choice because it is in the past tense and is consistent with the other verbs in the sentence.
Therefore, the corrected sentence using option C is:
"The 300 men trained diligently, screamed fiercely into battle, and fought valiantly."