How do drugs classified as SSRIs, such as Prozac, alter neuronal communication?

How do drugs classified as SSRIs, such as Prozac, alter neuronal communication?
 
  a. They increase the effectiveness of the neurotransmitter serotonin by stopping its diffusion from the synaptic cleft.
  b. They decrease the effectiveness of the neurotransmitter serotonin by returning it to the axon terminal for recycling.
  c. They increase the effectiveness of the neurotransmitter serotonin by blocking its reuptake into the axon terminal.
  d. They decrease the effectiveness of the neurotransmitter serotonin by facilitating its metabolism by neuronal enzymes.