In the Orsat apparatus for flue gas analysis, ammoniacal cuprous chloride, CuCl\(_2\) is specifically used to absorb CO. The CO acts as a Lewis base and forms a soluble coordination complex or adduct with the copper(I) ions.

A. \(\text{H}_{2}\) (Hydrogen) and C. \(\text{N}_{2}\) (Nitrogen) are generally unreactive with the solution and remain after the absorption process. These are typically determined by difference in a gas analysis experiment.

D. \(\text{CO}_{2}\) (Carbon dioxide) is absorbed by a different solution, typically an alkaline solution like potassium hydroxide (\(\text{KOH}\)), in the Orsat apparatus.