Ans is 'a' i.e. Salting out Ammonium sulfate precipitation is a method of protein purification by altering solubility of protein. The technique is known as salting out.Salting out:Proteins are purified using differences in protein solubility in varying salt concentrations.The solubility of proteins varies according to the ionic strength of the solution, and hence according to the salt concentration. Two distinct effects are observed: at low salt concentrations, the solubility of the protein increases with increasing salt concentration (i.e. increasing ionic strength), an effect termed salting in. As the salt concentration (ionic strength) is increased further, the solubility of the protein begins to decrease. At sufficiently high ionic strength, the protein will be almost completely precipitated from the solution (salting out).Since proteins differ markedly in their solubility's at high ionic strength, salting-out is a very useful procedure to assist in the purification of a given protein. The commonly used salt is ammonium sulfate, as it is very water soluble.The ammonium sulfate concentration is increased stepwise, and the precipitated protein is recovered at each stage.The precipitated protein is then removed by centrifugation and then the ammonium sulfate concentration is increased to a value that will precipitate most of the protein of interest whilst leaving the maximum amount of protein contaminants still in solution.This technique is useful to quickly remove large amounts of contaminant proteins, as a first step in many purification schemes. It is also often employed during the later stages of purification to concentrate protein from dilute solution following procedures such as gel filtration