To determine which quantity is expressed as force per unit area, we need to analyze the defined terms of each option:

• Work: Work is defined as the energy transferred when a force is applied over a distance. It is calculated as W = F imes d, where F is the force and d is the distance moved in the direction of the force. Thus, work is not related to force per unit area.
• Pressure: Pressure is indeed defined as force per unit area. It is mathematically expressed as P = \frac{F}{A}, where P is pressure, F is the applied force, and A is the area over which the force is applied. Therefore, pressure is the correct quantity.
• Volume: Volume measures the amount of three-dimensional space an object occupies. It is not related to force or area.
• Area: Area measures the extent of a two-dimensional surface and is not related to force.

Based on this analysis, the quantity expressed as force per unit area is Pressure.