Final answer:
The stimulation of a neuron by a neurotransmitter that opens K+ channels leads to hyperpolarization, making the neuron less likely to fire an action potential; the correct answers are 1) and 3), which combines into option 5) Both A and C.
Step-by-step explanation:
When a neurotransmitter stimulates a neuron and K+ channels open, potassium ions (K+) exit the cell. The loss of K+ leads to an increase in the negative charge inside the neuron compared to the outside, because these ions carry a positive charge with them out of the cell. This process is called hyperpolarization, making the inside of the cell more negative than the resting potential.
The neuron then enters the refractory period, where it is less likely to fire another action potential because the more negative internal environment is farther from the threshold needed to trigger an action potential. After this period, ion pumps work to restore the resting potential by pushing out excess K+, but until then, the neuron is in a hyperpolarized state.
Therefore, the effect of a neurotransmitter that opens K+ channels is that the neuron becomes hyperpolarized and is less likely to fire an action potential. Thus, the correct answers are 1) become hyperpolarized and 3) be less likely to fire an action potential, making 5) Both A and C the final answer for this question.