The correct answer is: A. Straight below centroidal axis.
A prestressed concrete beam is a beam that is reinforced with steel cables that are tensioned before the concrete is poured. This tensioning of the cables creates a compressive force in the concrete, which helps to resist the bending moment caused by the load.
The tendon is the steel cable that is used to prestress the concrete. The tendon can be placed either above or below the centroidal axis of the beam. If the tendon is placed below the centroidal axis, the compressive force in the concrete will be greater at the top of the beam than at the bottom. This will cause the beam to deflect upward, which is desirable in most cases.
If the tendon is placed above the centroidal axis, the compressive force in the concrete will be greater at the bottom of the beam than at the top. This will cause the beam to deflect downward, which is not desirable in most cases.
Therefore, the tendon should be placed below the centroidal axis of the beam if the loading is uniformly distributed.