Electronic configuration of a transition element $X$ in +3 oxidation State is $\left[Ar\right]3d^5$. What is its atomic number?

The correct option is B 26

In oxidation an element looses its electrons,and since given configuration is of an element which is already oxidized,actual configuration will be $\left[Ar\right]3d^{6}4s^2$.Hence atomic number will be $18+6+2=26.$

This is Fe having atomic number 26.