Answer: Contiguity
Explanation: Contiguity means that the conditioned stimulus must precede the unconditioned stimulus, and it must be a reliable indicator that the unconditioned stimulus is about to occur. In classical conditioning, the pairing of the conditioned stimulus (CS) with the unconditioned stimulus (US) needs to be close in time and consistently associated for effective conditioning to take place. This temporal relationship helps establish the association between the two stimuli and allows the conditioned stimulus to become a reliable predictor of the unconditioned stimulus.