The correct answer is d. both excitation and inhibition.
There is a process which is called summation and it refers to the process that determines whether or not an action potential (on postsynaptic neuron) will be generated by the combined effects of excitatory and inhibitory signals (from the presynaptic neurons). Depending on the sum total of inputs, summation may or may not reach the threshold voltage to trigger an action potential (firing of postsynaptic potential).
