The correct answer is: C. 50 to 100 mm.
The slump is a measure of the consistency of concrete. It is defined as the vertical distance that a concrete slump cone will spread when released from a height of 12 inches. A slump of 50 to 100 mm is recommended for foundations because it provides a good balance of workability and strength.
A slump that is too low will result in a concrete that is difficult to place and compact. This can lead to voids and honeycombing in the concrete, which can reduce its strength and durability. A slump that is too high will result in a concrete that is weak and prone to cracking.
The slump of concrete can be affected by a number of factors, including the type and amount of cement, the type and amount of aggregate, the water content, and the admixtures used. The slump should be adjusted as needed to achieve the desired consistency.
It is important to note that the slump is not the only factor that affects the strength of concrete. The type and amount of cement, the type and amount of aggregate, the water content, and the admixtures used also play a role. The slump should be used in conjunction with these other factors to achieve the desired strength and durability of concrete.