Magnetic moment $\mu =\sqrt{15}B.M.$ is the true for the pair:

The correct option is A

$C{o}^{2+},C{r}^{3+}$

As spin only magnetic moment is $\mu =\sqrt{n(n+2)}B.M.$, where n is the number of unpaired electrons.
According to the given magnetic moment, n = 3.
And among given ions $C{o}^{2+}{(d}^7),C{r}^{3+}\left(d^3\right)$ configuration has 3 unpaired electron .
So, the anwer is $C{o}^{2+}$,$C{r}^{3+}$