Which of the following will lead to the strongest conditioning?

Which of the following will lead to the strongest conditioning?

Select one:

a. The conditioned stimulus is presented just before the unconditioned stimulus.

b. The conditioned stimulus and unconditioned stimulus are presented at the same time.

c. A second stimulus is presented at the same time as an already existing conditioned stimulus for extra clarity.

d. The unconditioned stimulus is presented just before the conditioned stimulus.