Three-month-old Lionel loved to suck on his fingers. He was quite fond of putting his fingers in his mouth and then pulling them out only to look them over for a minute or two before putting them back in his mouth. He repeated this behavior over and over. Lionel's behavior with his fingers illustrates a:

primary circular reaction.

Explanation:

Primary Circular Reaction is the second substage of the Sensorimotor Stage of Cognitive Development.  

The Sensorimotor stage is the early stage in the Cognitive Development theory of Jean Piaget. In this stage of cognitive development, an infant learns to grow and develop through various trials.  

In Primary Circular Reaction stage of cognitive development, an infant learns to grow and develop through various trials. In this stage, an infant develops new schemas. A baby starts repeating the actions that they find pleasurable.  This stage occurs between the age of 1-4 months..

In the given case, Lionel's repeating behaviour of sucking his fingers is an example of Primary Circular Reaction. Lionel has developed a new schema of sucking his fingers which he finds pleasurable that he puts his fingers after every minute or two.  

So, the correct answer is Primary Circular Reaction.