ANS:B - tensile reinforcement

In a singly reinforced beam, the effective depth is the distance from the compression face (or edge) of the beam to the centroid of the tensile reinforcement. So, the effective depth is measured from its compression edge to the: Neutral axis of the beam. The neutral axis is the line within the beam's cross-section where the bending stress is zero. It divides the cross-section into two parts: compression zone above the neutral axis and tension zone below the neutral axis. The effective depth is measured from the compression face to the neutral axis, as this distance determines the lever arm for calculating the bending moment.