The answer is C.) surfing
Calisthenics are exercises consisting of a variety of motor movements and dont usually require any equipment
Walking is obvious that you don't need any equipment (clothes would be great, but are not required to walk XD)
Yoga is just a series of breathing control, simple meditation, and body posture exercises
Surfing requires a board to surf on. The board helps distribute the weight of the rider which helps he/she float and it also allows the waves to carry them across the water.
Hope this helps! :)