Filling of electrons in an orbitals takes place by using following rules:
Aufbau rule: Filling of electrons takes place in an energy levels in the increasing order of energy that is the one having lowest energy will fill first.
Hunds rule: No second electron will be filled in an orbital until each orbital occupies a single electron.
Pauli exclusion principle: No two electrons can have same spin in an orbital.
So here also also the rules must be applied in filling electrons , by Aufbau rule, energy level that is having lower energy in this case after 5s it will be 4d¹, will fill first. So the answer is 4d¹.
