Drugs that block the reuptake of serotonin will thereby increase the concentration of serotonin molecules in the?

The answer is "Synaptic gaps". 

Synaptic gapes otherwise called synaptic cleft is the little space in the middle of the cell membrane of an axon terminal and furthermore of the target cell with which this synapses. Synapses are intersections between neurons in which data is passed by neurotransmitters.

The answer should be d