What rule/principle States that electrons fill orbital from lowest energy to highest energy? A: Aufbau principle
B: Hund's rule
C: Pauli exclusion principle ​

The aufbau principle, from the German Aufbauprinzip (building-up principle), also called the aufbau rule, states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.

Solubility would be your answer.