Solve this :  Answer is option (3)

$$\begin{gathered} 20. \mathrm{If} \mathrm{y}=\frac{\mathrm{Ax}}{{\left[{\mathrm{B}}^2+{\mathrm{x}}^2\right]}^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}; \mathrm{where} \mathrm{A} \mathrm{and} \mathrm{B} \mathrm{are} \mathrm{positive} \mathrm{constants}; \mathrm{then} \mathrm{value} \mathrm{of} \mathrm{y} \mathrm{is} \mathrm{maximum} \mathrm{a} \mathrm{x} \mathrm{equal} \mathrm{to} : \\ \left(1\right) \mathrm{Zero} \left(2\right) \mathrm{B} \\ \left(3\right) \pm \frac{\mathrm{B}}{\sqrt{2}} \left(4\right) \frac{\mathrm{A}}{\mathrm{B}} \end{gathered}$$