Six-Year-Old Warren Watches as His Grandmother Puts Freshly Baked Cookies

Six-year-old Warren watches as his grandmother puts freshly baked cookies into a jar. The jar sits on the first shelf in grandma's cabinet, a height that Warren cannot yet reach. After his grandmother leaves the kitchen, Warren takes a chair, sets it in front of the counter, and climbs up to the cabinet to get a cookie. What is the operant in this example?

A) the cookie
B) grandma's presence
C) carrying the chair to the kitchen
D) Warren's age