Final answer:
The size of the mortar round for the 83D CM BN depends on the historical context of the unit. Historically, Chemical Mortar Battalions used a 4.2-inch (107mm) mortar. Contemporary units may use mortars ranging from 60mm to 120mm.
Step-by-step explanation:
The student is asking about the size of the mortar round that is fired by a military unit, specifically the 83D Chemical Mortar Battalion, which could indicate an interest in historical military equipment. However, without specific context, it is challenging to provide an accurate answer as the abbreviation '83D CM BN' could refer to a past or present unit, and different units may use different equipment. If '83D CM BN' refers to a historical unit, such as those used during World War II, U.S. Chemical Mortar Battalions commonly used the 4.2-inch (107mm) chemical mortar. Contemporary mortar units employ a variety of mortars that range from 60mm to 120mm in caliber, based on their tactical requirements.