For part (a), the context states that there are $n$ subshells in the $n^{th}$ shell. Thus, for $n = 4$, there are 4 subshells. For part (b), the maximum number of electrons in a shell with principal quantum number $n$ is $2n^2$. For $n = 4$, this equals 32 electrons. Since electrons in an orbital have opposite spins ($m_s = +1/2$ and $-1/2$), exactly half of the total electrons will have $m_s = -1/2$. Therefore, $32 / 2 = 16$ electrons.