Which of the following is a chemical substance secreted by an organ or gland?

The correct answer is hormone. Hormones are chemical substances that are produced and secreted by glands in the endocrine system. They travel through the bloodstream to various tissues and organs, where they exert their effects on different physiological processes such as growth, metabolism, reproduction, and mood regulation.

While enzymes, vitamins, and neurotransmitters also play important roles in the body, they differ fundamentally from hormones in terms of their origins and functions. Enzymes are proteins that catalyze biochemical reactions but are not classified as hormones since they are not secreted into the bloodstream for systemic effects. Vitamins are organic compounds essential for normal growth and nutrition, but they are not secreted by glands nor do they have hormonal functions. Neurotransmitters are chemical messengers that transmit signals between nerve cells, acting locally at synapses rather than traveling through the bloodstream like hormones do. This distinction highlights the unique role that hormones play in regulating various bodily functions through endocrine signaling.