In order to be able to solve this question we have to understand what Boyle's law is. According to Boyle's law; at constant temperature the pressure of a given mass of gas is inversely proportional to to its volume.
The Boyle's law shows us the relationship between the pressure and the volume. So, the Important thing to note hear is that if the volume in a container is decreased then the pressure will increase (and vice versa) due to the fact that as the volume decreases the particles in that container makes more collision which will make the pressure to increase.
Since, the piston is moveable it means that we can decrease and increase the volume in the cylinder. So, if the decrease the volume of the cylinder then we will have an increase in the pressure of the gas below the piston.
