A diamagnetic atom is one that has no unpaired electrons.
Going all the way to the 3p orbital, you have 18 electrons accounted for…
Cu2+ has 27 electrons; so 9 electrons left.
2 into 4s, 7 into 3d. Since 3d has 5 orbitals, 3 will be left unpaired.
Cu2+ is not diamagnetic.
Ni2+ has 26 electrons; so 8 electrons left.
2 into 4s, 6 into 3d. Since 3d has 5 orbitals, 4 will be left unpaired.
Ni2+ is not diamagnetic.
Cr3+ has 21 electrons; so 3 electrons left.
2 into 4s, 1 into 3d. The lone electron in 3d is unpaired.
Cr3+ is not diamagnetic.
Sc3+ has 18 electrons. All available orbitals are completely filled
Sc3+ IS diamagnetic.
And just for posterity,
Cr2+ has 22 electrons; so 4 electrons left.
2 into 4s, 2 into 3d. The two electrons are both unpaired.
Cr2+ is not diamagnetic.
easy, just test them in the flux capacitor. whichever produces 1.21 jiggawatts is your answer.