Assertion (A) Angular speed, linear speed and kinetic energy change...

As we know that, Torque is the moment of force and torque $(\tau)=\mathbf{r} \times \mathbf{F}$ where, $\mathbf{r}$ is distance between axis of rotation and force $(F)$. Also, $\tau=\frac{d L}{d t}$ and since, in space there is no external torque is applied so, $\tau=0\therefore \quad \Delta L=0=\text{ constant }$ Hence, Assertion (A) and Reason (R) both are true and Reason (R) is the correct explanation of Assertion (A).