The correct option is D Synapses
A synapse is a place where axon terminals (presynaptic neuron) of one nerve fibre lie close to the dendrites of the adjacent neurons (postsynaptic neuron). These junctions are called as synapses. These ensure one-way or unidirectional transmission of impulses, as neurotransmitters are released from presynaptic end only and their receptors are present on the postsynaptic end only.