Which of the following represents pressure ?

The correct answer is (b). Pressure is defined as force per unit area. So, the correct expression for pressure is $\frac{F}{A}$, where $F$ is the force and $A$ is the area.

Option (a) is incorrect because it represents density, which is mass per unit volume. Option (c) is incorrect because it represents the product of force and area, which is work. Option (d) is incorrect because it represents force per unit acceleration, which is momentum.