Isotopes are: A. only formed in laboratories.

B. found in nature.

C. are only theoretical.

D. found in the nuclear reactions instars but not on Earth.​

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.