Final answer:
At 4 months old, Mark would be showing primary emotions, which are the basic feelings that can be observed in early infancy.
Step-by-step explanation:
At 4 months old, Mark is in the sensorimotor stage of development, as described by Piaget. During this stage, infants like Mark learn about the world primarily through their senses and motor behavior. The right answer to the question, "Which of the following emotions would be showing at this age?" is D. Primary emotions. These emotions include basic feelings such as interest, distress, disgust, and happiness. Self-conscious and other-conscious emotions develop later, and there are no emotions classified as tertiary in the context of this discussion. Mark, at 4 months old, would not yet show self-conscious emotions, as self-awareness typically develops around 18 months of age, evidenced by the mirror test.