Why do gases exert pressure on the walls of the container?

The correct option is: Gas particles move randomly and strike the walls.

According to the kinetic molecular theory of gases, the particles (atoms or molecules) of a gas are in a constant state of rapid, random, and straight-line motion.
Because there are large spaces between these particles and negligible attractive forces between them, they move freely in all directions within the container.

As these gas particles move randomly, they continuously collide with one another and with the inner walls of the container.
Each collision of a gas particle with the wall exerts a tiny force on that wall.

Since there are billions of particles colliding with the walls every second, these individual collisions add up to produce a continuous, uniform force over the surface area of the container walls.
Pressure is defined as force per unit area:
$P=\frac{F}{A}$
Therefore, it is this constant bombardment of the container walls by randomly moving gas particles that exerts pressure on the walls.