The correct answer is A. 25 mm to 50 mm.
Slump is a measure of the consistency of concrete. It is defined as the average vertical distance that a concrete slump cone will spread when released from a height of 12 inches. A slump of 25 mm to 50 mm is recommended for mass concrete because it provides a good balance between workability and strength.
A slump of less than 25 mm will result in a concrete that is too stiff and difficult to place. A slump of more than 50 mm will result in a concrete that is too fluid and may not have adequate strength.
Mass concrete is a type of concrete that is used in large, heavy structures such as dams, bridges, and foundations. It is typically made with a high water-cement ratio, which makes it more workable and easier to place. However, a high water-cement ratio also results in a weaker concrete. Therefore, it is important to strike a balance between workability and strength when placing mass concrete.
A slump of 25 mm to 50 mm provides a good balance between workability and strength for mass concrete. It is workable enough to be placed easily, but it is also strong enough to support the weight of the structure.