The correct option is B 2Ma0g+a0
When the balloon descends with acceleration ao, total downward force is equal to Mao.
Mao=net downward force = weight of balloon - buoyant force
Mao=Mg−Vbρag.....(I)
Now lets say m mass is removed from the balloon, due to which the balloon rises with ao, So the net upward force this time is equal to (M−m)ao
(M−m)ao= net upward force = buoyant force- net weight of balloon
(M−m)ao=Vbρag−(M−m)g.....(II)
adding (I) and (I), we have
2Mao−mao=mg⇒2Mao=mg+mao=m(g+ao)⇒m=2Mao(g+ao)