The correct answer is A. 0.87.
The moment of resistance factor is a factor that is used to calculate the moment of resistance of a concrete beam. It is a function of the grade of concrete and the type of reinforcement used. For M 150 grade concrete (1 : 2 : 4), the moment of resistance factor is 0.87.
The moment of resistance of a concrete beam is the maximum bending moment that the beam can withstand without failing. It is calculated using the following formula:
$M_r = f_c’ b d \phi$
where:
- $M_r$ is the moment of resistance
- $f_c’$ is the compressive strength of the concrete
- $b$ is the width of the beam
- $d$ is the effective depth of the beam
- $\phi$ is the moment of resistance factor
The moment of resistance factor is a conservative value that is used to ensure that the beam will not fail. It is based on experimental data and takes into account the variability of concrete strength and the effects of reinforcement.
The moment of resistance factor is different for different grades of concrete and different types of reinforcement. For M 150 grade concrete (1 : 2 : 4), the moment of resistance factor is 0.87. This means that the moment of resistance of a concrete beam made with M 150 grade concrete is 0.87 times the moment of resistance of a beam made with an infinitely strong concrete.