Final answer:
A neurotransmitter is the chemical signal that diffuses across the synaptic gap between neurons to convey messages to the next cell, such as dopamine, which plays a role in neurological processes.
Step-by-step explanation:
The chemical signal that diffuses across the gap between adjacent neurons to convey a message to the next cell is known as a neurotransmitter. Neurotransmission occurs when neurotransmitter molecules are released by the axon terminal of a presynaptic neuron and bind to receptors on the dendrites of a postsynaptic neuron across the synaptic cleft, also known as the synaptic gap. This gap is typically around 20 nm (0.02 μ) wide, which allows for rapid changes in neurotransmitter concentration. An example of a neurotransmitter is dopamine, represented by the chemical formula C8H11NO2, which plays a key role in neurological processes. Therefore, the correct answer to the question is A) Neurotransmitter.