An IPSP is inhibitory because (a) it hyperpolarizes the postsynaptic membrane, (b) it reduces the amount of neurotransmitter released by the presynaptic terminal, (c) it prevents calcium ion entry...

An IPSP is inhibitory because

(a) it hyperpolarizes the postsynaptic membrane,

(b) it reduces the amount of neurotransmitter released by the presynaptic terminal,

(c) it prevents calcium ion entry into the presynaptic terminal,

(d) it changes the threshold of the neuron.