The correct answer is: C. 20 kg/cm2
Shear stress is a measure of how much force is being applied to a material in a direction parallel to its surface. It is calculated by dividing the force applied by the area over which it is applied.
The shear stress at which a beam will fail depends on a number of factors, including the material it is made of, its shape, and the load it is carrying. In general, however, most beams will fail if the shear stress exceeds 20 kg/cm2.
Option A (10 kg/cm2) is too low. Most beams will not fail if the shear stress is below this level.
Option B (15 kg/cm2) is a possibility. Some beams may be able to withstand a shear stress of 15 kg/cm2, but others may not.
Option C (20 kg/cm2) is the most likely answer. This is the shear stress at which most beams will fail.
Option D (25 kg/cm2) is too high. Most beams will fail if the shear stress exceeds this level.