Magnetic field at the centre due to semicircular loop lying in $x-y$ plane, $B_{x y}=\frac{1}{2}\left(\frac{\mu_{0} i}{2 R}\right) \quad$ negative $z$ direction.
Similarly field due to loop in $x-z$ plane,
$B_{x z}=\frac{1}{2}\left(\frac{\mu_{0} i}{2 R}\right)$ in negative $y$ direction.
$\therefore$ Magnitude of resultant magnetic field,
$B =\sqrt{B_{x y}^{2}+B_{x z}^{2}}=\sqrt{\left(\frac{\mu_{0} i}{4 R}\right)^{2}+\left(\frac{\mu_{0} i}{4 R}\right)^{2}}$
$=\frac{\mu_{0} i}{4 R} \sqrt{2}=\frac{\mu_{0} i}{2 \sqrt{2} R}$


Reason : Magnetic field energy density is proportional to square of current.
($A$) The maximum voltage range is obtained when all the components are connected in series
($B$) The maximum voltage range is obtained when the two resistors and one galvanometer are connected in series, and the second galvanometer is connected in parallel to the first galvanometer
($C$) The maximum current range is obtained when all the components are connected in parallel
($D$) The maximum current range is obtained when the two galvanometers are connected in series and the combination is connected in parallel with both the resistors