The correct answer is: C. 100 cm
Side face reinforcement shall be provided in the beam when depth of the web in a beam exceeds 100 cm. This is to prevent the web from buckling under the load. The web is the part of the beam that is perpendicular to the longitudinal axis of the beam. It is the most critical part of the beam, as it is the part that carries the most load. The web is made of concrete, which is a strong material, but it can buckle if it is not properly reinforced. Side face reinforcement is a type of reinforcement that is used to prevent the web from buckling. It is made of steel bars that are placed perpendicular to the web. The steel bars help to distribute the load evenly across the web, which prevents it from buckling.
The other options are incorrect because they are not the minimum depth of the web in a beam that requires side face reinforcement. Option A is 50 cm, which is too small. Option B is 75 cm, which is too small. Option D is 120 cm, which is too large.
I hope this explanation is helpful. Please let me know if you have any other questions.