Fully prestressed concrete beams are designed to resist all the vertical loads through the use of prestressing. Here is a detailed explanation of why option 'A' is the correct answer:

Prestressed Concrete Beams:
- Prestressed concrete beams are structural members that have been precompressed (prestressed) to withstand the tensile forces generated by the applied loads.
- This precompression is achieved by introducing high-strength steel strands, known as tendons, into the concrete beam and applying a pre-determined amount of force to them.
- The tendons are then anchored at the ends of the beam, creating a compressive force that counteracts the tensile forces induced by the loads.

Resisting Vertical Loads:
- Vertical loads refer to any forces or loads that act vertically on the beam, such as dead loads (the weight of the beam itself and any permanent attachments) and live loads (loads that are not permanently attached, such as the weight of people, furniture, or vehicles).
- Fully prestressed concrete beams are designed to resist all the vertical loads by transferring them to the ground or the supporting structure.
- This is achieved by the precompression force in the tendons, which creates a compressive stress within the concrete beam.
- The compressive stress counters the tensile stresses induced by the vertical loads, effectively neutralizing them and preventing any excessive deflection or failure of the beam.

Advantages of Prestressed Concrete Beams:
- Fully prestressed concrete beams offer several advantages over conventionally reinforced concrete beams.
- They have higher load-carrying capacity and can span longer distances without the need for additional supports.
- The precompression force helps to reduce or eliminate cracking in the concrete, resulting in a more durable and long-lasting structure.
- Prestressed concrete beams also provide a higher degree of design flexibility, allowing for innovative and efficient structural solutions.

Conclusion:
- In summary, fully prestressed concrete beams are designed to resist all the vertical loads by utilizing the precompression force in the tendons.
- This precompression creates a compressive stress within the beam, which counteracts the tensile stresses induced by the loads.
- By effectively neutralizing the vertical loads, prestressed concrete beams provide a robust and reliable structural solution.