When assigning electrons to orbitals, which would be the most likely 'address' for the next electron following the 5s2 electron? 5s3 5p1 4d1 5d1

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.

The answer is number 3, 4d1.