The fact that the 2s orbital would fill with electrons before the 2p orbital would accept electrons is an application of:

the Aufbau principle

Hund's rule

the Pauli exclusion principle

Schrödinger's equation

I think the correct answer from the choices listed above is the first option. The fact that the 2s orbital would fill with electrons before the 2p orbital would accept electrons is an application of the Afbau principle. It states that, hypothetically, electrons orbiting one or more atoms fill the lowest available energy levels before filling higher levels.