The answer is c.CS; US.
In classical conditioning, a new behaviour is learned via association. The stage before conditioning involves: 1. unconditioned stimulus (US) which produces an unconditioned response, and 2. neutral stimulus (NS) which has no effect on a person and produce a response only when paired with unconditioned stimulus. During conditioning, neutral stimulus (NS) and unconditioned stimulus (US) pair and neutral stimulus becomes a conditioned stimulus (CS). In other words, the ns becomes a CS after it reliably signals the impending occurrence of the US.