Tension bars in a cantilever beam must be enclosed in the support up to Ld.

Explanation:
- Cantilever beam: A beam supported at one end and carries a load at the other end or distributed along its length.
- Tension bars: A component used to resist tension or pulling forces in a structure.
- Support: The fixed point where the beam is attached or held in place.
- L: The length of the tension bar.
- d: The diameter of the tension bar.
- φ: The allowable stress in the tension bar.

When a tension bar is used to resist the pulling forces in a cantilever beam, it must be enclosed in the support up to a certain length to ensure that it remains in place and functions properly. This length is determined by several factors, including the length and diameter of the bar and the allowable stress in the material.

The correct answer is option A, which states that the tension bars must be enclosed in the support up to Ld. This means that the entire length of the tension bar must be supported by the structure up to the point where it connects to the beam. This ensures that the bar can effectively resist the forces acting on the beam without slipping or becoming dislodged.

Other considerations when designing tension bars for cantilever beams include:
- The diameter of the bar should be sufficient to resist the forces acting on the beam.
- The material used for the bar should have a high tensile strength and be resistant to corrosion.
- The allowable stress in the material should be determined based on the desired safety factor for the structure.