The ideal gas law states that the pressure of a gas is directly proportional to its temperature and inversely proportional to its volume. In this case, since the volume and pressure are the same for all three cylinders, the temperature is the only factor that will affect the mass of oxygen in each cylinder.

The higher the temperature, the lower the density of the gas. This means that cylinder c, which has the highest temperature, will contain the lowest mass of oxygen. Cylinder a, which has the lowest temperature, will contain the highest mass of oxygen.

Therefore, the cylinder that contains the largest mass of oxygen is cylinder a.