Which of the following is a disadvantage of using hydroelectric power such as the
Hoover Dam?
A
B
They produce an abundance of
electricity
The energy is renewable.
They have a negative impact on the
environment.
с
D They do not produce carbon dioxide.

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