The correct answer is option 'D', which states that the shear strength can be ensured in a beam by providing stirrups. In this answer, we will explain why stirrups are essential for ensuring shear strength in a beam.

Shear Strength in Beams:
When a beam is subjected to vertical loads, it experiences internal forces that cause it to bend. These internal forces create shear stresses in the beam. Shear strength refers to the ability of the beam to resist these shear stresses and prevent failure.

Importance of Stirrups:
Stirrups are an essential component of reinforced concrete beams. They are typically made of steel and are placed perpendicular to the main reinforcement bars (rebars) along the length of the beam. Stirrups help to enhance the shear strength of the beam in the following ways:

1. Confinement of Concrete: When a beam is subjected to shear forces, the concrete tends to crack along diagonal planes. Stirrups act as confining elements that prevent the cracks from opening up further. They hold the concrete in place and enhance its ability to resist the shear stresses.

2. Transfer of Shear Forces: Stirrups help in transferring the shear forces from the concrete to the rebars. By providing additional reinforcement in the form of stirrups, the shear forces are distributed more evenly throughout the beam, reducing the chances of failure.

3. Increase in Effective Depth: The presence of stirrups increases the effective depth of the beam. This is because the stirrups occupy space within the beam, effectively increasing the distance between the compressive and tensile forces. A higher effective depth results in a higher moment of inertia, which in turn increases the shear capacity of the beam.

4. Preventing Excessive Deflection: In addition to enhancing shear strength, stirrups also help in preventing excessive deflection of the beam. They provide lateral support to the rebars, reducing their tendency to buckle under the applied loads. This improves the overall structural performance and stability of the beam.

Conclusion:
In summary, stirrups are an essential component for ensuring shear strength in a beam. They provide confinement to the concrete, transfer shear forces to the rebars, increase the effective depth of the beam, and prevent excessive deflection. By incorporating stirrups in the design and construction of beams, engineers can ensure the safe and efficient performance of structures.