When chromium loses two electrons, its configuration changes to a. [ar]4s13d5. b. [ar]4s1. c. [ar]4s13d4. d. [ar]3d4.

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.