a)    Wetting agents: Wetting agent is a surfactant that when dissolved in water, lower the contact angle and reduce the surface tension of a liquid. According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it or no. which is the wettability between liquids and solids. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface and vice versa.

To more clarification example of behavior of water and mercury in test tube can be taken. The surface of liquid water (meniscus) has a concave shape because water wets the surface and creeps up the side. The surface of Mercury has a convex shape it does not wet glass because the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass.