An endothermic reaction will start when the required energy is received from the environment or solution.
A. ΔH
B. kinetic
C. activation
D. thermal

Is there a diagram/picture you can provide? it seems this question goes along with a

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.

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