Final answer:
The baby hitting the mobile more frequently in response to the sound it produces is an example of positive reinforcement, even though this exact option is not provided in the question.
Step-by-step explanation:
The behavior of the baby hitting the mobile more frequently after hearing the twinkling sound is the result of positive reinforcement. This concept isn't explicitly listed in the provided options, but it aligns with the process of increasing the likelihood of a behavior by following it with a pleasurable outcome (the sound of the bells). Neither proprioceptive stimuli, phylogeny, negative reinforcement, nor ontogeny are correct in explaining why the baby strikes the mobile more often after experiencing the pleasant sound. Proprioceptive stimuli relate to the body's ability to sense its movement and position in space, phylogeny refers to the evolutionary development of a species, negative reinforcement involves increasing behaviors by removing an undesirable stimulus, and ontogeny concerns the individual development or life history of an organism.