Final answer:
A hormone is A) A chemical message carried in the bloodstream, produced by glands of the endocrine system, acting on target cells throughout the body to regulate physiological processes.
Step-by-step explanation:
The best description of a hormone is A) A chemical message carried in the bloodstream. Hormones are special chemical messenger molecules that are secreted by glands in the endocrine system. These molecules are released in response to certain stimuli and travel through the bloodstream to reach various parts of the body. Once at their destination, hormones bind to specific receptors on target cells, initiating a response that can influence physiological processes such as growth, development, and behavior.
Hormones contrast with neurotransmitters because, unlike neurotransmitters that have a local and rapid effect, hormones have widespread and longer-lasting effects.
Notably, hormones can also be found in plants, where they help the plant cells communicate and respond to environmental stimuli, just like in animals.