Answer:
In this case, picking up the infant served as a positive reinforcer for crying.
Explanation:
In operant conditioning, reinforcement is a strategy used to increase the likelihood of the appearance of a desired behavior by adding a reward or a favorable stimulus, this stimulus is called a reinforcer.
In this case, although Mandisa probably didn't want to increase the appearance of the crying behavior, she gave her daughter a positive reinforcement every time she cried, by picking her up. This way, picking her up acted like the stimulus that reinforced the behavior.