Organs .

A. perform functions that tissues cannot carry out on their own
B. are made of only one type of tissue
C. only exist in animals
D. are less complex than the tissues which make them

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.