A spherical shell of 1 kg mass and radius $R$ is rolling with angular speed $\omega$ on horizontal plane (as shown in figure). The magnitude of angular momentum of the shell about the origin $O$ is $\frac { a } { 3 } R ^ { 2 } \omega$. The value of $a$ will be\\
(1) 2\\
(2) 3\\
(3) 5\\
(4) 4