Which mortar round is typically not used during training because of cost?

The choice of service ammunition as the correct answer is based on the context of training operations in the military. Service ammunition refers to the live, combat-ready rounds that are used in actual field conditions. These rounds are typically costly and therefore are not allocated for routine training exercises, where the goal is to provide soldiers with the necessary practice and skills without incurring high expenses.

In contrast, other types of mortar rounds such as high-explosive (HE) rounds and illuminating (ILLUM) rounds have their specific training uses, but they also have established training alternatives. Additionally, tube practice/fire training rounds (TP/FRTR) are specially designed for training purposes to simulate the firing of live rounds while being less expensive. Therefore, in a training environment, the goal is to balance cost with effective training, which is why service ammunition is generally reserved for combat operations rather than routine training scenarios.