Final answer:
Muscles are responsible for heartbeat, muscle tone, and moving bones. These functions are facilitated by different types of muscles: cardiac for heartbeat, and skeletal for muscle tone and movement of bones.
Step-by-step explanation:
The functions of muscles include various vital activities in the human body. Heartbeat is driven by cardiac muscle, which is unique to the heart and is responsible for pumping blood throughout the circulatory system.
Muscle tone, which refers to the continuous and passive partial contraction of muscles, or the muscle's resistance to passive stretch during resting state, is primarily maintained by skeletal muscles. Skeletal muscles are also involved in moving bones, enabling not only voluntary movements like walking and running, but also stabilizing the body and maintaining posture. Given that muscles play a role in all these activities, the correct answer to the student's question would be D. all of the above.