Final answer:
The most suitable nursing diagnosis for the described sacral wound is 'Impaired skin integrity' due to the current state of damage to the skin including a foul odor, purulent drainage, and necrotic tissue.
Step-by-step explanation:
The most appropriate nursing diagnosis for a patient admitted to the hospital with a sacral wound that has a foul odor, purulent drainage, and necrotic tissue in the center, which measures 4 cm in circumference by 2 cm deep, is b. Impaired skin integrity. This diagnosis directly addresses the current state of the wound, indicating that the skin and tissues are damaged and require corrective treatment to heal and prevent further deterioration. Options such as risk for infection are also relevant, but the presence of purulent drainage and necrotic tissue indicates that an infection is likely already present, and thus the skin integrity is unequivocally impaired.