Factor of safety against sliding of a slope, is the ratio of

ANS:D - both (a) and (b).

Both a and b means the same thing (almost) , a is for purely cohesive soils, their Shear stress is equal to cohesion. Strength- it is the resistance against the gradual loading.
Shear stress- it is the resistance between two objects. Stress is reaction force induced when force is applied. Its increas as applied force increat but upto its limit (ita strength) and strength is maximum reaction force soil can induced to withstand applied force. Stress is the force applied, strength is the force the material can resist. When the shear stress reaches the shear strength of the material, it breaks.