The correct answer is C. Neutral axis of the beam.
The neutral axis is the imaginary line in a beam where the tensile and compressive stresses are equal. It is located at a distance from the compression edge that is equal to the effective depth of the beam. The effective depth is the distance from the compression edge to the centroid of the tension reinforcement.
The tensile edge is the edge of the beam that is in tension. The tensile reinforcement is the reinforcement that is placed in the tension zone of the beam. The longitudinal central axis is the axis that runs along the length of the beam and passes through its centroid.
The effective depth is important because it determines the maximum bending stress that the beam can withstand. The larger the effective depth, the greater the bending stress that the beam can withstand.