Final answer:
The primary purpose of a motorcycle top box is to provide c) increased storage capacity for riders to carry items.
Step-by-step explanation:
The primary purpose of a motorcycle top box, which is specifically designed for motorcycles, is c. Increased storage capacity. This accessory is intended to provide riders with additional space to carry personal items, equipment, or supplies conveniently while riding. While a top box may have a secondary effect on aesthetics, aerodynamics, or even fuel efficiency, those are not its primary purposes.