Question

the water flowing over stone in a stream or river makes them round and smooth mainly due to (1) . fluid friction, (2). sliding friction, (3). static friction, (4). rolling friction

Answer 1

I think rolling friction.

Answer 2

Option 2. Sliding Friction - is the correct answer.

This is because the water molecules slides over the rock surface.

Reasons other are not:

Option 1: Fluid friction is the other name of viscosity. Viscosity is between liquid - liquid surface.

Option 3: water is not static hence this is not applicable.

Option 4: water molecules is not rolling over the stones.