Mercury does not wet glass, wood or iron because

The correct option is: Cohesive force is greater than adhesive force.

To understand why mercury does not wet glass, wood, or iron, we need to look at the intermolecular forces acting at the contact surface between the liquid and the solid. These forces are:
1. Cohesive Force: The force of attraction between molecules of the same substance (in this case, between mercury atoms themselves).
2. Adhesive Force: The force of attraction between molecules of different substances (in this case, between mercury atoms and the molecules of glass, wood, or iron).

Whether a liquid wets a solid surface depends on the relative strengths of these forces:
- If the adhesive force is greater than the cohesive force, the liquid molecules are pulled toward the solid surface more strongly than they are pulled toward each other. This causes the liquid to spread out and wet the surface (as water does on clean glass).
- If the cohesive force is greater than the adhesive force, the liquid molecules are pulled more strongly toward each other than toward the solid. Consequently, the liquid minimizes its contact with the solid by pulling itself into droplets and does not wet the surface.

Because the cohesive force within mercury (driven by strong metallic bonding) is significantly greater than the adhesive force between mercury and glass, wood, or iron, mercury maintains its spherical droplet shape and does not wet these materials.