For part (a), the total number of orbitals in the $n=4$ shell is $n^2 = 16$. Since each orbital accommodates one electron with $m_s = -1/2$, there are 16 such electrons. For part (b), $n=3$ and $l=0$ corresponds to the 3s subshell. For $l=0$, the only permitted value of $m_l$ is 0, meaning there is only one orbital, which can hold a maximum of 2 electrons.