Final answer:
The primary difference between the trousers of junior enlisted soldiers and NCOs is the presence of stripes on the trousers for NCOs, which reflects historical military practices for easily identifying rank and maintaining discipline.
Step-by-step explanation:
The most noticeable difference between the Army Blue Service Uniform trousers worn by Soldiers in the ranks of PVT through SPC and those worn by NCOs in the ranks of CPL through SMA is d) Trousers stripes. This distinction is a traditional way of visually differentiating the rank classes within the military uniform etiquette.
It is important to note that the need for uniform distinction has historical roots stemming from the necessity for military police (MPs) to easily identify and regulate the behavior of soldiers, especially when they were off-base on leave. This practice also reflects broader military cultural norms and the strict regulation of appearance and conduct among service members.