In a two dimensional motion of a particle, the particle moves from point $A$, position vector $\mathbf{r}_{1}$ to point $B$, position vector $\mathbf{r}_{2}$. If the magnitudes of these vectors are respectively, $r_{1}=3$ and $r_{2}=4$ and the angles they make with the $x$-axis are $\theta_{1}=75^{\circ}$ and $15^{\circ}$, respectively, then find the magnitude of the displacement vector
(a) 15
(b) $\sqrt{13}$
(c) 17
(d) $\sqrt{15}$