What is a flame test used for? A. To determine the ionization energies of electrons
B. To identify an unknown sample by its melting point
C. To identify an unknown sample by its emission spectrum
D. To remove impurities from a sample through burning

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

The answer is the third choice

The answer would be solvent