During the laboratory preparation of chlorine gas, hydrogen chloride (HCl) gas is also produced as an impurity. The gas mixture is passed through a wash bottle containing water, in which the HCl gas is highly soluble and is absorbed, while chlorine is only fairly soluble. This step removes the majority of the HCl impurity from the chlorine gas stream.

A. dilute it: The primary purpose is the removal of a specific impurity (HCl), not general dilution. The gas is passed through water to remove a contaminant, not just to reduce its concentration.

C. neutralize it: While water can be used with a base to neutralize an acid, in this specific setup, water primarily acts as a solvent for the highly soluble HCl gas to separate it from the less soluble chlorine gas.

D. absorb the Cl₂ gas: Chlorine gas is only fairly soluble in water, and the goal is to collect the chlorine, not absorb it.

Therefore, the correct answer is to absorb HCl gas - option B