- A Hormones, secreted into the bloodstream by glands
- B Enzymes, which catalyze specific biochemical reactions
- C Neurotransmitters
- D Vitamins, obtained only through dietary intake
Correct answer: C. Neurotransmitters
Explanation: Neurotransmitters are chemical messengers released at synapses. Acetylcholine and noradrenaline are the most common.
Concept context
Brain, spinal cord, neurons, reflex arcs, and sense organs.