Which of the following is a secondary reinforcer

A)  a hug 
B)  money 
C)  a reinforcer that comes after the operant behaviour 
D)  a reinforcer that is less salient than another reinforcer

A secondary reinforcer is a conditioned or learned reinforcer that gains reinforcing value through a pairing with a primary reinforcer, such as money which serves as an indirect means of obtaining primary reinforcers like food or shelter. A hug (A) is a primary reinforcer as it provides immediate physical comfort and social contact. Choice C is incorrect as it describes any type of reinforcer regardless of whether it is a primary or secondary reinforcer. Choice D is incorrect as salience refers to the perceived importance or relevance of a reinforcer and is not a defining characteristic of secondary reinforcers.