Final answer:
The force that caused the boy to slow down when he hit a rough patch while sliding on the frozen lake is friction.
Step-by-step explanation:
The force that caused the boy to slow down when he hit a rough patch while sliding on the frozen lake is friction.
Friction is the force that opposes the motion of an object when it slides against another surface. The rough patch on the frozen lake creates more surface contact between the boy's sled and the ice, increasing the friction and causing him to slow down.
Other forces listed such as gravity, air resistance, and magnetic force do not directly affect the boy's speed when hitting a rough patch on the frozen lake.