ANS:C - ion-exchange

Deaeration, which involves the removal of dissolved oxygen (O2) from water, is typically achieved through the process of absorption. In absorption processes for deaeration, water is brought into contact with a suitable absorbing medium that has a high affinity for oxygen. The oxygen molecules dissolve into the absorbent, thereby reducing the concentration of dissolved oxygen in the water. Common absorbents used for deaeration include:

1. Chemical Scavengers: Certain chemicals, such as sodium sulfite or sodium bisulfite, can react with dissolved oxygen to form non-volatile compounds, effectively removing oxygen from the water.
2. Vacuum Deaeration Towers: These towers utilize a vacuum to lower the partial pressure of oxygen above the water, facilitating the release of dissolved oxygen from the water into the gas phase.
3. Membrane Deaeration: Membrane systems can selectively remove dissolved gases, including oxygen, from water by passing the water through semi-permeable membranes.

Rectification, ion-exchange, and adsorption are not typically used for deaeration purposes. Rectification refers to the separation of components in a liquid mixture by distillation, ion-exchange is a process for removing ions from water by exchanging them with ions of similar charge on a resin, and adsorption involves the attachment of molecules onto the surface of a solid material, which may not be effective for removing dissolved gases like oxygen from water.