If $(1+x)^{15}=a_0 +a_1 x +.....+ a_{15} x^{15},$ then $\sum \limits_{r=1}^{15} r \large\frac{a_r}{a_{r_1}}$ is equal to :

Question

\[\begin {array} {1 1} (a)\;110 & \quad (b)\;115 \\ (c)\;120 & \quad (d)\;135 \end {array}\]