Final answer:
Neurons are activated by neurotransmitters, which are chemical messengers released at synapses and bind to receptors on postsynaptic cells, initiating an action potential.
Step-by-step explanation:
Neurons are activated by neurotransmitters (option a). All types of senses, including vision and other sensory input, undergo sensory transduction by converting a stimulus into an electrical signal through the nervous system. This electrical signal can activate neurons. However, transmission of a nerve impulse from one neuron to another, or to another type of cell, occurs across a synapse.
At a synapse, synaptic vesicles in the axon terminal of the presynaptic neuron release chemical messengers called neurotransmitters into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic neuron. Neurotransmitters can be either excitatory or inhibitory, meaning they can increase or decrease the likelihood of an action potential in the postsynaptic neuron.