A researcher distributes a personality inventory to 20 study participants. She then divides the participants into two separate groups based on their ostensible performance on the personality inventory: pulligans and nulligans. However, there truly is no difference between the participants in each group. She then finds that participants tend to define themselves based on traits that pulligans and nulligans apparently tend to possess. This example best illustrates:

social identity theory.

Explanation:

The social identity theory of the suggestion that an organisation can decide to change the behaviour of individuals if it can bring modifications to their self identity.

It gives specifications and predictions to the situations where people might see themselves as one person or as members of a group.

From this question, we can see that the participants to find themselves based on the group traits to which they belonged. Those that were shared into pulligand defined themselves based on those traits. And nulligans defined themselves based on the nulligan traits.