(c) Hind brain | Cerebellum
The textbook explicitly states: "activities like walking in a straight line, riding a bicycle… are possible due to a part of the hind-brain called the cerebellum." Cerebellum controls precision of voluntary actions and maintains posture and balance — not the medulla (which controls involuntary actions like blood pressure) or cerebrum (main thinking part).
Source: Chapter 6, Section 6.1.2 – Human Brain