Answer :

Correct answer is : 16
For n = 4
l = 0, 1, 2, 3
Possible subshells are
4s, 4p, 4d and 4f
Number of electron have S = +1/2
4s = 1
4p = 3
4d = 5
4f = 7
Total number of electron with S = +1/2 for n = 4 = 16