a. How many sub-shell are associated with `n = 4`?
b. How many electron will be present in the sub-shell having `m_(s)` value of `-1//2` for `n = 4`?

Explanation

n=4 (Given )
for a given value of "n" , "l" can have values from zero to (n-1)
l=0,1,2,3
thus four sub - shells are associated with n=4 , which are s,p ,d and f.
(b) number of orbitals in the `n^(th) shell = n^(2)`
for n =4
number of orbitals =16
if each orbital is taken fully, then it will have 1 electron with `m_(s) ` value of `-1/2`