Answer:
S-delta
Explanation:
S-delta: The term "S-delta" can also be written as SD, and is defined as the stimulus due to which a particular behavior is not being reinforced because of its presence.
While conducting a discrimination training, an animal generally responds to a particular stimulus which seems similar to the S-delta, and finally, after a period the animal tends to stop responding to S-delta and it is being distinguished.
In the question above, the statement signifies the use of 'S-delta'.