The magnetic moment of a transition metal is $\sqrt{15}\mathrm{BM}$. Therefore, the number of unpaired electrons present in it is ____.

Magnetic moment $\sqrt{\mathrm{n}(\mathrm{n}+2}$

So put the value in the formula 
$$\begin{gathered} \sqrt{15} =\sqrt{\mathrm{n}(\mathrm{n}+2)} \\ \mathrm{Squaring} \mathrm{both} \mathrm{side} \\ 15 = \mathrm{n}(\mathrm{n}+2) \\ 15 = {\mathrm{n}}^2+2\mathrm{n} \\ {\mathrm{n}}^2 +2\mathrm{n} -15 =0 \\ \mathrm{n}(\mathrm{n}-3) +5 (\mathrm{n}-3) = 0 \\ (n+5)(n-3)= 0 \\ So , \mathrm{n} = -5 , +3 \\ -\mathrm{ve} \mathrm{value} \mathrm{can} \mathrm{be} \mathrm{neglected} \\ \mathrm{n} = +3 \end{gathered}$$
Hence , the correct answer is +3.