Final answer:
Option (d), Digestion is not a function of the skin. The skin's principal functions include thermoregulation, sensation, protection, excretion, and production of vitamin D, with digestion occurring in the gastrointestinal tract.
Step-by-step explanation:
The principal functions of the skin include thermoregulation, sensation, protection, excretion, and vitamin D synthesis. Among the options provided, digestion is NOT a function of the skin. When the body is too warm, the skin contributes to heat regulation by producing sweat.
The sensation function relates to the skin's ability to detect changes in the environment such as heat, pressure, and pain. Excretion is a process by which the skin helps in the removal of waste through sweat. However, digestion is a process that occurs within the gastrointestinal tract and is not a function of the skin.