Which statement best describes a hormone?

A hormone is best described as a substance secreted by the glands in the body. Hormones play a vital role in regulating various physiological processes, including growth, metabolism, and mood, by traveling through the bloodstream to target organs and tissues. They are produced by endocrine glands such as the pituitary, thyroid, and adrenal glands, making option A accurately highlight the essence of what hormones are and how they function within the body.

The other options refer to different concepts: muscle contractions pertain to the movement of muscles and are not related to hormone secretion. The mention of a chemical substance used in medications does not specify the unique characteristics of hormones, which are specific types of signaling molecules. Finally, food additives are components added to enhance food properties but do not encapsulate the definition of hormones or their regulatory functions within the body.