Answer
A. Its volume is decreased
Step-by-step explanation
If we compress a gas (decrease) without changing its temperature, the average kinetic energy of the gas particles remains the same.
At a constant temperature, the kinetic energy of particles remains the same. If the volume is reduced at a constant temperature, then the number of particles in unit volume or area increases. If there is an increased number of particles in the unit area then it increases the frequency of collisions per unit area.
At constant temperature, the smaller the volume of the container, the larger the pressure.