5. Dr. Duarte is studying bullying behavior in children.

Every day, he goes to the local playground at 3 p.m., sits on the sidelines, and records the number of times one child bullies another, the sex of the children involved in the bullying, and the duration of the bullying. Dr. Duarte is using which research method in his study?

a. An experiment

b. A case study

c. A naturalistic observation

d. A quasi-experiment