Answer:
B. Operant conditioning
Step-by-step explanation:
In psychology, the term operant conditioning refers to a form of learning by reinforcements and punishments.
According to this, a reinforcement is an stimulus that increases the probability that a certain behavior takes place again in a future, so, when the individual is presented with this stimulus, the behavior will be more likely repeated in the future and it will happen more often.
In this example a child smiles and gains attention, the reinforcement would be the gaining of attention, which would be the stimulus that is making the child to begin to smile more often. Gaining attention is something desirable and pleasant for the child so it's a reinforcement for the conduct of smiling.
Therefore this is an example of b. operant conditioning.