Final answer:
In Liebig's method, the Cu gauge is placed near the exit of the combustion tube to remove halogens and assist in the conversion of sulfur dioxide when a compound contains these elements. So, the correct option is the D. Cu gauge.
Step-by-step explanation:
In Liebig's method for the estimation of carbon (C) and hydrogen (H), when a compound also contains halogens and sulfur (S), the correct material to place near the exit of the combustion tube is Cu gauge (Option D). Cu gauge or copper gauze is used because copper effectively removes any halogens by forming copper halides and also catalyzes the conversion of sulfur dioxide (SO2) to sulfur trioxide (SO3), which can then be trapped and measured separately.
By using the Cu gauge, one can ensure that the measurements for carbon and hydrogen are not interfered with by the halogens or sulfur present in the compound. The combustion analysis technique is a crucial part of this process and helps in accurately determining the elemental composition of an unknown compound.