For several weeks Allen had to clean the men's restroom at the restaurant where he worked. The task always made him nauseated. He has since gone on to better things, but still cannot walk by the door to a men's restroom without becoming slightly queasy

For Allen, the door to the men's room has become__________
a) an unconditioned stimulus.
b) an unconditioned response.
c) a reflexive stimulus.
d) a conditioned stimulus.

d