(solved)Question 2.67 of NCERT Class XI Chemistry Chapter 2

$\displaystyle \underline{\underline{\text{Part (a)}}}$

subshells in principal quantum number $\displaystyle n$ are $\displaystyle n^2$

$\displaystyle \therefore $ subshells for $\displaystyle n = 4$ are $\displaystyle 4^2 = 16\:\underline{Ans}$

$\displaystyle \underline{\underline{\text{Part (b)}}}$

Each subshell can have only one electron of
spin $\displaystyle m_s = -1/2$

$\displaystyle \therefore $ total electrons of spin -1/2 will also be $\displaystyle 16\:\underline{Ans}$