Explanation:

When two pieces of a glass plate are placed one upon the other with a little water in between them, they become difficult to separate easily. This is primarily due to the phenomenon of surface tension.

Surface Tension:
Surface tension is the property of a liquid that enables it to resist external force or minimize its surface area. It is caused by the cohesive forces between the liquid molecules at the surface.

Surface Tension and Water:
Water molecules have strong cohesive forces due to hydrogen bonding between them. This cohesive force creates a thin film on the surface of water, which gives rise to surface tension.

Formation of Surface Tension in the Glass Plate:
When the two glass plates are placed one upon the other with a little water in between, the water molecules at the interface between the glass plates experience strong cohesive forces. These cohesive forces cause the water molecules to form a thin film and create surface tension.

Effect of Surface Tension:
The surface tension between the two glass plates acts as a binding force, making it difficult to separate them easily. The cohesive forces between the water molecules and the glass surfaces are stronger than the adhesive forces between the two glass plates. As a result, the surface tension holds the two glass plates together, resisting separation.

Other Options:
- Inertia (option A): Inertia is the resistance of an object to changes in its motion. In this scenario, inertia does not play a significant role in the difficulty of separating the glass plates.
- Pressure (option B): Pressure is the force exerted per unit area. While there may be some pressure exerted between the glass plates due to the weight of the water, it is not the primary factor in their difficulty of separation.
- Viscosity (option C): Viscosity is a measure of a fluid's resistance to flow. While water does have viscosity, it is not the main factor responsible for the difficulty in separating the glass plates.

Therefore, the correct answer is option D, surface tension, as it is the primary factor that makes it difficult to separate the two glass plates placed with a little water in between them.

The property of surface of a liquid that allows it to resist an external force due to cohesive nature of its molecules is surface tension.... so .. surface tension create repulsive force when plates with little water between them is separated. .. hence it makes harder to separate them easily.... so the answer becomes d)surface tension