Carl
Level 15

Tax help for military filers

You'll select E-5, as that's the standard pay grade for a cadet at military academies such as West Point.

I don't know if cadets get the same type of LES as active/enlisted do. But if you do, then looking at it from the top left, the third box over is "Grade", and that's your paygrade. Again, I would expect it to be E-5.