Final answer:
Hash marks on the left sleeve of a dress uniform represent years of service, with each hash mark usually indicating four years of service.
Step-by-step explanation:
Hash marks on the left sleeve of a dress uniform represent years of service. Each hash mark typically indicates a certain period of time served, most commonly four years, which can differ slightly among branches of the military. The hash marks are an embodiment of the individual's dedication and commitment to their service in the armed forces. Therefore, the correct answer to the student's question is a) Years of service. It's essential to distinguish these service stripes from other types of military insignia, such as badges for the number of deployments, combat awards, or rank, which are designated through different symbols and placements on the uniform.