Final answer:
Hormones are the biomolecules that specifically cause organisms to change their activities in response to stimuli, by binding to target cell receptors and inducing responses within the organism.
Step-by-step explanation:
Something that causes an organism to change its activity is a hormone. A hormone is released in response to a stimulus, travels through the body, and binds to receptors on target cells to induce a response. This response is part of a signaling pathway that alters the activity of the organism. Hormones can affect growth, development, metabolism, and many other processes within an organism. Enzymes, while also proteins, act as biological catalysts to speed up chemical reactions and are not directly responsible for changing an organism's activity in response to stimuli. Antibodies protect against foreign agents and genes contain the instructions for protein synthesis. It is hormones that have the specific role of signaling and regulating the activity of cells.