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

The correct option is B 26
$X^{3+}=\left[Ar\right]3d^5=23$ electrons. As $X^{3+}$ is formed by the loss of 3 electrons from $X$, ${}^{\prime }{X}^{\prime }$ will have 26 electrons.
$\therefore$ Atomic number of $X=26$.