The answer is D. Sickness.
Disease, illness, and health are all related concepts, but they have different meanings.
A disease is a condition that affects the body and can cause illness. Illness is a state of being unwell, and health is a state of being free from disease or illness.
Sickness is a more general term that can refer to either disease or illness. It can also be used to describe a feeling of being unwell, even if there is no underlying medical condition.
In the context of the question, the word that is different from the rest is “sickness” because it is the only word that can refer to both disease and illness. The other words, “disease,” “illness,” and “health,” are all more specific terms that refer to only one of these concepts.