The moment of a beam refers to the ability of the beam to resist bending or twisting moments. It is a measure of the beam's resistance to bending under an applied load. The moment of a beam is determined by its section modulus, which is a property of the beam's cross-sectional shape.

Moment of Inertia:
The moment of inertia is a measure of an object's resistance to changes in its rotational motion. It depends on the distribution of mass around the axis of rotation. The moment of inertia of a beam is related to its section modulus through the equation: moment of inertia = section modulus * area. Therefore, the moment of inertia is not directly related to the moment of a beam.

Stress:
Stress is a measure of the internal forces within a material caused by an external load. It is defined as the force per unit area. Stress is not directly related to the moment of a beam.

Strain:
Strain is a measure of the deformation of a material caused by an external load. It is defined as the change in length per unit length. Strain is not directly related to the moment of a beam.

Coefficient of Elasticity:
The coefficient of elasticity, also known as Young's modulus, is a measure of a material's stiffness. It relates the stress in a material to the strain it experiences. The coefficient of elasticity is not directly related to the moment of a beam.

Half the Depth:
The depth of a beam is one of the factors that determine its section modulus. The section modulus is directly proportional to the depth of the beam. However, half the depth of a beam is not directly related to the moment of the beam.

Therefore, the correct answer is option B, moment of inertia. The moment of a beam is defined as its section modulus multiplied by the moment of inertia of its cross-sectional shape. The section modulus takes into account both the shape and size of the beam, while the moment of inertia accounts for the distribution of mass around the axis of rotation.