Two short bar magnets of equal dipole moment $M$ are fastened perpendicularly at their centres, figure. The magnitude $T_{N}$ of resultant of two magnetic field at a distance $d$ from the centre on the bisector of the $P$ right angle is
(a) $\frac{\mu_{e}}{4 m} \frac{2 \sqrt{2} M}{d^{7}}$
(b) $\frac{\mu_{0}}{4 \pi} \frac{2 M}{d^{3}}$
(c) $\frac{\mu_{e}}{4 \pi} \frac{M}{d^{4}}$
(d) $\frac{\mu_{0}}{4 \pi} \frac{\sqrt{2} M}{d^{3}}$