It is more difficult to walk on a sandy road than on a concrete road because

In physics, friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. When walking, the friction between the ground and the feet is crucial for maintaining stability and propulsion.

Option A: While it's true that sand is soft and concrete is hard, the question is specifically asking about the difficulty of walking on each surface, which relates more to friction than hardness.
Option B: Sand has a lower coefficient of friction compared to concrete. This means that there is less resistance between the feet and the sand, making it more difficult to walk on a sandy road as compared to a concrete road where the friction between the feet and concrete is greater, providing better traction and stability.
Option C: This is the opposite of the correct explanation. If the friction between sand and feet were more than that between concrete and feet, walking on sand would be easier, not more difficult.
Option D: The texture of the surfaces (grainy sand versus smooth concrete) might play a role in perception, but the primary factor affecting difficulty in walking is the friction between the surfaces and the feet, as explained in option B.

Therefore, the correct answer is Option B: the friction between sand and feet is less than that between concrete and feet.