In PAGE, a layer of organic solvent is introduced on top of the resolving gel to (a) Facilitate entry of proteins (b) Increase production of free radicals (c) Decrease the rate of acrylamide polymerization (d) Prevent the entry of oxygen

The answer is (d) Prevent the entry of oxygen.

In Polyacrylamide Gel Electrophoresis (PAGE), a layer of organic solvent, such as butanol or isopropanol, is often added on top of the resolving gel. This is done to create a flat surface, but more importantly, to prevent the entry of oxygen. Oxygen can inhibit the polymerization of acrylamide, which is necessary for the formation of the gel. Therefore, the organic solvent layer acts as a barrier to oxygen, ensuring the proper polymerization of acrylamide and the successful formation of the gel.