Baby Jacob's mother is more likely to pick him up when he smiles than when he is fussy. As a result, the frequency of Jacob's smiles has increased. This is an example of

Operant conditioning.

Explanation:

Operant conditioning refers to a  learning method that obtains through rewards and punishments to measure change in behavior. By operant conditioning, there is a correlation made between an action and a reason for that behavior. According to this theory, actions followed by positive effects are likely to repeat in future scenarios. As per the question, Jacob associates his smile and his mother's reaction of picking him up and therefore perfect example of operant conditioning.

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