The correct answer is: The reinforced concrete beam is safe in shear.
The safe shear stress for a reinforced concrete beam is 5 kg/cm2. The shear force on the beam is 6 t/cm2, which is less than the safe shear stress. Therefore, the beam is safe in shear.
The shear force is the force that acts parallel to the cross-section of the beam. The lever arm is the distance from the neutral axis to the outermost fiber of the beam. The safe shear stress is the maximum shear stress that the beam can withstand without failing.
If the shear force is greater than the safe shear stress, the beam will fail in shear. This can cause the beam to crack or break. To prevent this, the beam can be reinforced with steel bars. The steel bars will help to distribute the shear force and prevent the beam from failing.
In this case, the shear force is less than the safe shear stress, so the beam is safe in shear. The beam does not need to be redesigned.