The correct options are
B The gas can be liquified on applying pressure at constant temperature
C In low pressure region, Z can be greater than one for a gas
D For each gas, in very high pressure region, Z is always greater than one
The critical temperature is the highest temperature at which the gas starts condensing in a liquid. When the temperature of a gas is below its critical temperature, and pressure is applied at a constant temperature, the gas is liquified.
The compressibility factor z=PVnRT. It is always greater than one for any gas in the very high-pressure region. This is because the pressure term appears in the numerator. In the low-pressure region, it can be greater than one for gas. This is possible at low temperature and high volume.
Hence, the correct answer is options A, C and D.