The correct answer is D. All the above.
A. Cracks appear on the plastered surface in the form of hair cracks. This is because the plaster is not properly bonded to the backing or the backing is not strong enough to support the weight of the plaster.
B. In brick work, the efflorescence is removed by applying a solution of zinc sulphate and water. Efflorescence is a white powder that forms on the surface of brickwork. It is caused by the leaching of salts from the brickwork into the water that is used to mix the mortar. The zinc sulphate in the solution reacts with the salts to form a precipitate that is then washed away with the water.
C. Excessive thermal variations in the backing or plaster causes the plaster to fall. This is because the plaster expands and contracts with changes in temperature. If the backing or plaster is not flexible enough to accommodate these changes, it will crack and eventually fall off.
It is important to note that these are just some of the reasons why plaster can crack or fall off. There are many other possible causes, and it is important to consult with a professional if you are experiencing problems with your plaster.