Electronic configuration of cromium is
Cr-[Ar]4s¹3d⁵
When cromium loses two electrons it becomes Cr⁺².
So its electronic configuration becomes,
Cr⁺²-[Ar]3d⁴
One electron will go from 4s orbital and one electron will go from 3d orbital.
So the answer here is D. [Ar]3d⁴ -because after losing 2 electrons electronic configuration of cromium becomes [Ar] 3d⁴.
Smog
formed by mixture of smoke and fog

Actually Welcome to the concept of Bond Formation.
Answer is :-
B.) O and Cl
These both are Non-Metals with high Electronegativity nature, hence none of them can share electrons to foram any bond to eventually form a Electro valentines Compound.