Answer
Based on $I=mr^{2}$, angular momentum is conserved or $I\omega$ is fixed
Now as the disk is heated, it expands and the radius increases.
As a result the inertia $I$ will increase. Since the angular momentum is fixed, with increase of $I$, $\omega$ will decrease. So, the rotational speed will decrease.
Work Step by Step
Based on $I=mr^{2}$, angular momentum is conserved or $I\omega$ is fixed
Now as the disk is heated, it expands and the radius increases.
As a result the inertia $I$ will increase. Since the angular momentum is fixed, with increase of $I$, $\omega$ will decrease. So, the rotational speed will decrease.
