Final answer:
According to Rescorla and Wagner, the association between a conditioned stimulus and an unconditioned stimulus is strongest when the association is d. variable
Step-by-step explanation:
In classical conditioning, the association between a conditioned stimulus (CS) and an unconditioned stimulus (US) is strongest when the association is surprising. According to Rescorla and Wagner, the strength of the association depends on the degree of unpredictability or surprise of the US following the CS.
For example, if a conditioned stimulus consistently predicts the occurrence of an unconditioned stimulus, the association may become weaker because the organism learns to expect the outcome. On the other hand, if the association is variable and the unconditioned stimulus is sometimes present and sometimes absent, the association may be stronger because the organism is constantly surprised by the outcome.
Therefore, the correct answer is d. variable because a variable association between the CS and US leads to stronger associative learning.