B

Step 1: Understand the concept of cohesive and adhesive forces.
- Cohesive forces are the intermolecular forces between molecules of the same substance (in this case, mercury).
- Adhesive forces are the intermolecular forces between molecules of different substances (in this case, mercury and glass, wood, or iron).

Step 2: Analyze the behavior of mercury with respect to wetting.
- Wetting is the ability of a liquid to maintain contact with a solid surface. This is dependent on the balance of cohesive and adhesive forces.
- If cohesive forces are greater than adhesive forces, the liquid will not wet the surface of the solid.

Step 3: Apply this understanding to the options given.
- Option A states that cohesive force is less than adhesive force, which is incorrect.
- Option B states that cohesive force is greater than adhesive force, which correctly explains why mercury does not wet these materials.
- Option C incorrectly addresses the angle of contact being less than 90°, as this is a condition for wetting.
- Option D incorrectly states that the cohesive force is equal to adhesive force, which does not explain non-wetting.

Conclusion: Therefore, the correct answer is Option B: Cohesive force is greater than adhesive force.