Answer: His
Explanation: His, brother is a singular indefinite pronoun; therefore, the singular form his agrees with the antecedent
If whipped is not a part of the underlined sentence, we can rule out a verb, leaving "through the massive trees" which is an adjective phase, as it is describing the "massive" trees.
Given the conditions, the closest answer would be conventions. Word choice is using colorful language such as heavy adjectives and replacement of better words. Sentence fluency deals with fragment sentences or run-ons. Ideas deal with the main topic of an article or essay, etc.