An inhibitory synapse sits on the axon hillock. An excitatory synapse producing a potential of exactly the same size sits far out on a distal dendrite. Why does the inhibitory one have the greater influence on whether the neuron fires?

C. Because postsynaptic potentials shrink as they spread.

Postsynaptic potentials are passive and leak away as they spread, so two equal potentials are no longer equal by the time they reach the axon hillock where the decision is taken. It is why inhibitory synapses cluster on the soma, anatomy is doing arithmetic, and position counts alongside amplitude.