a presynaptic neuron delivers a rapid volley of subthreshold stimuli causing the postsynaptic neuron to fire.?

b/ temporal summation

I would pick b. Temporal summation is an effect generated by a single neuron as a way of achieving action potential. Summation occurs when the time constant is sufficiently long, a fraction of a second, and the frequency of rises in potential are high enough that a rise in potential begins before a previous one ends. The amplitude of the previous potential at the point where the second begins will algebraically summate, generating a potential that is overall larger than the individual potentials. This allows the potential to reach the threshold to generate an action potential.

To expand on the second person's answer you would be measuring the change in charge around the membrane of the postsynaptic neuron. Neurotransmitters would either make it more positive leading to an action potential or more negative which would make an action potential less likely. Since nerotransmitter affect channels or pumps that change ion locationsone of these two things is going to happen. The other reason to measure the change in action potential is that all you need is a volt meter. Otherwise you would need a device that could measure microscopic amounts of the 50+ neurotransmitters that occur in the body. Some of which we haven't even discovered yet.