A pig trained by the Brelands to pick up large wooden coins and deposit them in a wooden "piggy bank" learned quickly to do so for food rewards, but eventually a. reverted to rooting for the coins instead, in accordance with its natural approach to food. b. chose to wallow in the mud instead, in order to cool off. c. became too bored with the repetition of the task to continue. d. reverted to washing the coins instead, in accordance with its natural approach to food. ond to stimuli that are similar to an original conditioned stimulus is called a. generalization. b. habituation. c. aggregation. d. sensitization.

   d. reverted to washing the coins instead, in accordance with its natural approach to food.

Explanation:

Timulus discrimination is the tendency to lack a conditioned response to a new stimulus that is similar to the original conditioned stimulus. A higher order conditioning occurs when a neutral stimulus comes to act as a conditioned stimulus when combined with another stimulus that already evokes a conditioned response.

The correct answer for the blank is-

Morphology.

Explanation:

Morphology can be described as a branch of biology, which deals with the study of structure and forms (morphological/ physical traits such as height and color of individual, eye color, hair color and shape etc) of organisms without considering their function/roles.

Sometimes appearance of an organism is a key to determine a taxon.

Therefore, in this aspect, scientists use morphology that corresponds to appearance of organisms.