The correct answer is A. 37 cm.
The lever arm is the distance from the centroid of the tension reinforcement to the neutral axis of the beam. The neutral axis is the imaginary line in the beam where the stresses are zero. The centroid of the tension reinforcement is the point where the entire area of the tension reinforcement can be considered to be concentrated.
The modular ratio is the ratio of the modulus of elasticity of steel to the modulus of elasticity of concrete. The modulus of elasticity is a measure of how much a material will deform under stress.
The effective depth of the beam is the distance from the top of the beam to the centroid of the tension reinforcement.
The permissible working stresses in steel and concrete are the maximum stresses that can be applied to the steel and concrete without causing failure.
The lever arm can be calculated using the following formula:
Lever arm = â(Modular ratio x Effective depth)
In this case, the modular ratio is 18, the effective depth is 46 cm, and the lever arm is therefore:
Lever arm = â(18 x 46) = 37 cm
The other options are incorrect because they do not take into account the modular ratio, the effective depth, or the permissible working stresses in steel and concrete.