Psychologists once believed that any stimulus (whether a taste, sight, or sound) could serve equally well as a conditioned stimulus. this belief was challenged by research demonstrating the importance of

Biological Constraints is the correct answer.

Biological constraints are tendencies that are develeped through evolutionary history which induce members of a species to learn some associations that are naturally adaptive more easily than others. Any stimulus (such as sight, sound or taste) are believed to serve as a conditioned stimulus.