Boyle's law gives the relationship between pressure of a gas and volume of gas.
It states that at constant temperature, pressure of gas is inversely proportional to volume of gas.PV = kÂ
where P - pressure  V- volume and k - constantÂ
P1V1 = P2V2Â
since the gas occupies the volume of the container, volume of gas is taken as volume of container.
100kPa x 5.0 L = 50 kPa x V
V = 10.0 LÂ
Answer is C.