The strain which causes a material to twist is known as a. stress b. tension c. shear d. compression

The answer is c. shear

When earthquakes occurs, rocks are constantly exposed to forces known as stress which cause deformation of rocks that tend to bend, twist, or fracture them. The respond to stress that result to changes in shape, size, or volume are referred to as strain. 

If stress act equally from all directions, the applied force is a uniform stress. However, if stress is not equivalent from all directions then it is a differential stress. Moreover, there are various kinds of differential stress namely, tensional stress that causes the rock to stretch, the compressional stress that squeezes the rock and the shear stress which is a consequence of slippage, twisting or transformation of rocks.