Question

Classical conditioning is most successful when the neutral stimulus begins: long before the unconditioned stimulus begins. Immediately after the unconditioned stimulus begins. Just before the unconditioned stimulus begins. X at exactly the same time that the unconditioned stimulus begins.

Answer 1

I believe the answer is: just before the unconditioned stimulus begins.

By pairing the neutral stimulus with unconditioned stimulus on a short period of time, the subject would automatically perceive that both of them are strongly correlated.  This speed up the creation of conditioned response that intended toward the subjects,.

Answer 2

Classical conditioning is most successful when the neutral stimulus begins just before the unconditioned stimulus begins.