The correct option is D Acceleration of M1 will be equal to zero, which that of M2 will be M1−M2M2g upward
As M1>M2, so tension in thread connecting the block B and support C is (M1−M2)g. When this thread is burnt, this tension disappears/ In this case tension in spring is M1g and it s elongated. So, tension in string connecting A and B is M1g. Hence, resultant force on A remains zero because tension in string is balanced by tension in spring. The block B has a net upward force (M1−M2)g, so initial acceleration of block B is
(M1−M2)gM2
Thus, initial acceleration of M1 is zero and that of M2 is (M1−M2)gM2 upward.