Question

which action is not an example of work?
picking up a box off the floor
holding a tray in the cafeteria line
raising a barbell over your head
pushing a box across the floor

Explanation:

Work is defined as the product of force and displacement in the direction of the force ($W = F\times d$). When picking up a box, raising a barbell, or pushing a box, there is a displacement in the direction of the applied force. However, when holding a tray, there is no displacement in the direction of the force (the force is countering gravity, but the tray doesn't move vertically).

Answer:

holding a tray in the cafeteria line