ANS:D - all the above.

1. Its dimensions are not decided from the diagonal tensile stress: In prestressed concrete members, the prestressing tendons are placed under tension before the concrete is cast. This induces compressive stresses in the concrete, which counteract the tensile stresses induced by external loads. As a result, the dimensions of prestressed concrete members are not solely governed by the diagonal tensile stress, as is the case in conventionally reinforced concrete members. Instead, prestressing allows for the optimization of member dimensions based on other factors such as serviceability requirements, deflection control, and structural efficiency.
2. Large size of long beams carrying large shear force need not be adopted: Prestressed concrete members can effectively resist large shear forces due to the prestressing tendons providing internal resistance. This allows for the design of slender members with reduced cross-sectional dimensions compared to conventionally reinforced concrete members. Therefore, prestressed concrete members can accommodate large shear forces without the need for excessively large member sizes, resulting in more economical designs.
3. Removal of cracks in the members due to shrinkage: Prestressed concrete members experience reduced cracking compared to conventionally reinforced concrete members due to the compressive stresses induced by prestressing. These compressive stresses help to offset the tensile stresses induced by shrinkage and external loads, thereby reducing the likelihood and extent of cracking. As a result, prestressed concrete members often exhibit improved durability and aesthetic appearance, with fewer visible cracks.

Therefore, all of the above statements are valid reasons why prestressed concrete members are preferred over conventionally reinforced concrete members. They offer enhanced structural performance, economy in design, and improved durability, making them suitable for a wide range of structural applications. Thus, the correct answer is all the above.