Final answer:
In order to prevent the reinforcement of responses in SΔ, the appropriate contingency to add is extinction.
Step-by-step explanation:
In order to keep the onset of SD from reinforcing responses in SΔ, one needs to add a/an extinction contingency. Extinction is the process of weakening and eventually eliminating a conditioned response by repeatedly presenting the conditioned stimulus without the unconditioned stimulus. By repeatedly presenting the conditioned stimulus alone, the organism learns that it does not predict the arrival of the unconditioned stimulus, leading to a decrease in the conditioned response.