Which term is used to indicate a substance needed to maintain health?

The term that indicates a substance needed to maintain health is "replacement." In medical and health contexts, a replacement substance is one that restores or supplements components in the body that may be lacking due to various reasons, such as nutritional deficiencies or metabolic disorders. This could include vitamins, minerals, hormones, or even fluids that help ensure the body functions properly and maintains overall health.

In comparison, diagnostic refers to tests or procedures that help identify a disease or condition rather than substances necessary for health maintenance. Prophylactic involves measures or treatments used to prevent illness or disease, such as vaccinations or certain medications used to avert health problems rather than to maintain health. Palliative refers to care that focuses on providing relief from symptoms and improving the quality of life for patients with serious illnesses, rather than maintaining health.

Understanding these distinctions clarifies why "replacement" is the most appropriate choice for indicating a substance required for sustaining health.