In the transition elements, the incoming electron occupies (n-1)d sublevel in preference to:
As we learned in concept
Aufbau Principle -
In the ground state of the atoms, the orbitals are filled in order of their increasing energies.
According to Aufbau principle the electrons are filled in the order of energy of the orbitals. The orbital that is filled after (n-1)d is np by order of energy.