The correct answer is option 'D': Increased towards the centre of the span.

Explanation:
Shear stirrups are used in reinforced concrete (RC) beams to enhance the shear resistance of the beam. They provide confinement to the concrete in the shear zone and help in preventing the shear failure of the beam.

When designing shear stirrups in an RC beam, it is important to consider the variation of the shear force along the span of the beam. The shear force is usually maximum at the supports and decreases towards the center of the span. Therefore, the spacing of shear stirrups is increased towards the center of the span to provide adequate shear resistance.

Here is a detailed explanation of why the spacing of shear stirrups is increased towards the center of the span:

1. Shear force distribution:
In a simply supported RC beam, the shear force distribution is triangular, with the maximum shear force occurring at the supports and decreasing linearly towards the center of the span. This distribution is due to the bending moment distribution in the beam.

2. Shear failure mechanism:
Shear failure in RC beams typically occurs in a diagonal tension zone, known as the shear zone, which extends from the support towards the center of the span. The diagonal cracks develop in this zone when the shear force exceeds the shear resistance of the concrete.

3. Need for shear stirrups:
To prevent shear failure, shear stirrups are provided in the shear zone of the beam. They resist the diagonal tension forces and help in transferring the shear force from the beam to the stirrups.

4. Stirrup spacing:
The spacing of shear stirrups is determined based on the maximum shear force and the shear resistance of the stirrups. As the shear force is maximum at the supports and decreases towards the center of the span, the spacing of shear stirrups is increased towards the center of the span.

- At the supports, where the shear force is maximum, the spacing between the stirrups is kept smaller to provide sufficient resistance against the shear force.
- As we move towards the center of the span, where the shear force is lower, the spacing between the stirrups can be increased as the shear force is less and the demand for shear resistance is lower.

This variation in the spacing of shear stirrups along the beam ensures that there is adequate shear resistance throughout the span to prevent shear failure.

Because maximum shear act in beam near the column there required shear stirups while shear decreases towards the center of beam.