The correct answer is: A. Kept constant throughout the length.
Stirrups are used in reinforced concrete beams to prevent the concrete from splitting under tension. They are typically made of steel and are placed perpendicular to the longitudinal reinforcement. The spacing of stirrups is typically kept constant throughout the length of the beam, although it may be increased near the ends of the beam to provide additional reinforcement.
Option B is incorrect because the spacing of stirrups is typically kept constant throughout the length of the beam. Option C is incorrect because the spacing of stirrups is typically kept constant throughout the length of the beam, and there is no need to increase it at the ends of the beam. Option D is incorrect because the spacing of stirrups is typically kept constant throughout the length of the beam, and there is no need to increase it at the centre of the beam.