Answer: Option D. Increased at the center of the beam
Spacing of stirrups in a rectangular beam is increased at the center of the beam.
In a beam the maximum shear force is usually developed at the supports compared to the center. To resist the high shear force, maximum shear resistance is required at the supports, and at the center minimum shear resistance is enough. The spacing between the stirrups is inversely proportional to the shear resistance. Therefore where shear resistance is high the spacing between the stirrups decreases and vice versa.
As the shear resistance required at the center is less, the spacing of stirrups in a rectangular beam is increased at the center of the beam.