An electron can enter into the orbital when ?

The correct option is B Value of (n+l) is minimum
According to Aufbau principle electrons first, occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled.

The energy of an orbital depends on the $(n+l)$ value. Higher is the $(n+l)$ higher is the energy of the orbital. thus electron will first enter into an orbital with the minimum value of $(n+l)$.