According to the kinetic theory of gases, the pressure exerted by a gas on the walls is measured as :

The correct option is A rate of change of momentum imparted to the walls per second per unit area

According to Newton's second law:

$Force=rateofchangeofmomentum$

$F=\frac{dp}{dt}$

Particles impart momentum on the wall.

And the rate of change of momentum is the force exerted on the wall.

Pressure is force per unit area.

Hence pressure exerted by gas on the wall is the rate of change of momentum imparted to the walls per second per unit area.

Hence option A is correct.