Answer: 2nd US Cavalry (redesignated in 1861 as 5th US Cavalry).
Source:
https://tshaonline.org/handbook/online/articles/qls03
it seems the answers were a bit all over the map, so We will defer to hoosier on how to proceed with this question.
Edit - As I suspected, the question was asking for the identification of the pre-Civil War U. S. Army regiment of which 16 men who ultimately became Civil War generals were members.
The tricky part is the fact that, as pointed out in the link provided in post # 35, there were six regular mounted regiments in the U. S. Army in 1861: the 1st and 2nd Dragoons, the Mounted Rifles, and the 1st, 2nd, and 3rd Cavalry. These regiments were redesignated in order of seniority, and as a result of that redesignation, the 2nd Dragoons became the 2nd Cavalry and the 2nd Cavalry became the 5th Cavalry, both of which regiments still exist today.
The regiment which contained the 16 future Civil War generals (including four of the eight Confederate full generals) was the pre-war 2nd Cavalry, which became the 5th Cavalry. The pre-war 2nd Dragoons, which became the 2nd Cavalry, did not contain 16 future Civil War generals.
Therefore, I have to consider the 2nd Dragoons to be an incorrect answer.
hoosier