This is where something called Graham's Law applies. Grahams law says that the rate of effusion is inversely proportional to the square root of the molar mass of a gas. What that means is that the less the molar mass of a gas is, the less quickly the gas effuses (effusion being the rate at which a gas can travel through a small hole). A great way to think about this is that effusion is thought about as though the gas is traveling through a small hole, so smaller particles would be able to go through it with greater ease than would a large particle. I don't know what particular sentences the question asks for are, but the answer should be that gas A (molar mass 4) has the greatest effusion rate, gas B (molar mass 16) has the second fastest effusion rate, and gas C (molar mass 32) has the slowest effusion rate.
Polyhalogen derivatives are given when Phenol is treated with bromine water, in which all the H-atoms present at the o- and p- positions are substituted by Bromine with respect to the -OH group.
