For which of the following, the Limiting friction is minimum?\t500g\t250g\t50g\t300g

Limiting friction depends on the following factors:

The amount of Friction that can be exerted between two surfaces is limited and if the forces acting on the body are made sufficiently great, motion will occur. Hence, we define limiting friction as the friction which is exerted when equilibrium is on the point of being broken by one body sliding on another. The magnitude of limiting friction is given by the following three laws.
Law1
The ratio of the limiting friction to the Normal reaction between two surfaces depends on the substances of which the surfaces are composed and not on the magnitude of the Normal reaction.

This ratio is usually denoted by [\mu] .
Thus if the Normal reaction is R, the limiting friction is [\mu\:R]
For given materials polished to the same standard [\mu] is found to be constant and independent of R.
[\mu] is called The Coefficient of friction


Law2
The amount of limiting friction is independent of the area of contact between the two surfaces and of the shape of the surfaces, provided that the Normal reaction is unaltered.
Law 3
When motion takes place the direction of friction is opposite to the direction of relative motion and independent of velocity. The magnitude of the force of friction is in a constant ratio to the Normal reaction but this ratio may be slightly less than when the body is just on the point of moving.