To understand the description of the athlete as "pugnacious," Gentry would benefit most from using a dictionary. A dictionary would provide the definition of the word "pugnacious," helping him grasp the meaning and context in which it is used to describe the boy.
While a thesaurus could offer synonyms and give an idea of similar words, it does not provide the direct definition. A glossary might be helpful if it contained specific terms related to boxing or youth activities but wouldn't specifically explain "pugnacious." Therefore, the dictionary is the best choice for understanding that specific term.