A disk like reel with a mass-less thread unrolls while falling vertically downwards. The acceleration of its free fall is??

If there is no mass in the thread how will acceleration take place???

If mass of disc(cylinder like reel) is considered only. then
Torque equation, calculate torque due to tension about the centre, T X R = I x angular acceleration
Force equation, mg – T = M a
Calculate, and find the acceleration..

IF>> mass of the string is given and as it is decreasing, then concept would be something different....

Good Luck