In classical conditioning the ns becomes a after it reliably signals the impending occurrence of the
a. us; cs
b. ur; cr
c. cs; us
d. cr; ur

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. 

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