Final answer:
Repeated pressure on the skin causes thickening and results in a callus, which is a protective response to mechanical stress and minor injuries. Bedsores, stretch marks, eczema, and acne have varied causes and are different from calluses. The correct option is D.
Step-by-step explanation:
Continued, repeated pressure on any part of the skin can cause it to thicken and develop into a callus. Calluses are formed when the basal stem cells in the stratum basale of the skin proliferate more frequently to protect the underlying tissues from abrasive pressure.
This process is an adaptive response to minor injuries or constant mechanical stress, such as from ill-fitting shoes, writing for long periods, or playing a string instrument. Other skin conditions like bedsores, stretch marks, eczema, and acne have different causes and characteristics.
Bedsores, for example, result from prolonged, unrelieved pressure leading to tissue death, while acne is caused by clogged sebaceous glands.