Explanation

(c)
According to aufbau principle, electrons enter into orbitals according to their energy. The electrons first enters into orbital having lesser value of $(n + l)$. If the value of $n + l$ is same for two orbitals then the electron will first enter into orbital having lesser value of $n$.
$n = 5,\ l = 0\ \therefore n + l = 5 + 0 = 5$
For other,
$n = 3,\ l = 2\ \ \ \ \ \therefore n + l = 3 + 2 = 5$
∵ Both of the orbitals have same value for $n + l$.
∴ Electron will enter into orbital having lower value of $n$.
∴ Electron will enter into $n = 3,\ l = 2$ orbital.