Low activity of 'serotonin' neurotransmitter has been associated with

Depression

The correct answer is Option (3) → Depression

When an electrical impulse reaches a neuron’s ending, the nerve ending is stimulated to release a chemical, called a neuro-transmitter. Studies indicate that abnormal activity by certain neuro-transmitters can lead to specific psychological disorders. Anxiety disorders have been linked to low activity of the neurotransmitter gamma aminobutyric acid (GABA), schizophrenia to excess activity of dopamine, and depression to low activity of serotonin.