The correct answer is: C. 45°
Shear reinforcement is a type of reinforcement that is used in concrete beams to resist shear forces. Shear forces are forces that tend to cause a beam to slide along its own axis. Without shear reinforcement, concrete beams are susceptible to diagonal tension cracks, which can cause the beam to fail.
The angle of the diagonal tension cracks in an R.C.C. beam not provided with shear reinforcement is approximately 45° to the horizontal. This is because the shear force causes the concrete to be in tension on the diagonal, and the concrete is strongest in tension when it is loaded perpendicular to its fibers.
The other options are incorrect because they are not the approximate angle of the diagonal tension cracks in an R.C.C. beam not provided with shear reinforcement.