(a) Yes, the change of water into water vapour can be reversed. The process used here is condensation, which is the cooling of vapours to give liquid state water.

(b) Yes, the change of water vapour into water can be reversed. This can be done by evaporation. Water vapour, once changed to water, can be heated again to give vapours or gaseous state of water.

(c) Yes, Melting of ice changes its state from solid form to liquid form. On cooling, or refrigeration, this water can be changed back to the ice. Therefore, it is a reversible change.

(d) No, Changing of curd into milk is a chemical change which involves the use of bacteria. The milk once made into curd, cannot be brought back in milk or liquid form.