Bcz as shown in this question that angular momentum conservation has been applied about the centre of mass but when we stay at the centre of mass there may be an application of friction on the plane surface so that it may produce a torque so how will the conservation of angular momentum will be applied about centre of mass ??
but if we do the same thing about any point on the plane then we can apply it because the friction force then passes through the particular point
but what if we try to solve about the centre of mass because if we don't know about the friction that maybe acting or not be acting so how can we know that because if we stay at the centre of mass and we apply the conservation of angular momentum that means that the torque has been zero about COM but here the friction force is non zero so how can we do this
Sir I m facing a lot of problem during such questions