The degree of workability refers to the ease with which a concrete mixture can be mixed, transported, placed, and compacted. It is an important property of concrete as it affects its strength, durability, and overall quality. The slump test is commonly used to measure the workability of concrete.

The slump test involves filling a standard slump cone with freshly mixed concrete and then removing the cone to measure the settlement or "slump" of the concrete. The slump is measured in millimeters and indicates the consistency and flowability of the concrete.

In this question, the correct answer is option 'C' which states that the degree of workability is medium when the value of slump in mm is 50 – 100. Let's understand why this is the correct answer.

- Slump Range for Medium Workability:
When the slump value falls within the range of 50 – 100 mm, it is considered to have medium workability. This means that the concrete mixture is neither too stiff nor too fluid. It has a good balance of flowability and cohesiveness, making it suitable for a wide range of construction applications.

- Significance of Medium Workability:
Medium workability is desired in many construction scenarios where the concrete needs to be easily placed and compacted while maintaining its shape and stability. It allows for efficient pouring, spreading, and consolidation without excessive bleeding or segregation.

- Applications of Medium Workability Concrete:
Medium workability concrete is commonly used in various construction projects such as foundations, slabs, beams, columns, and walls. It is suitable for both horizontal and vertical placements, including flatwork and formwork.

- Factors Affecting Workability:
The workability of concrete is influenced by several factors, including the water-cement ratio, aggregate properties, cement content, admixtures, and the use of supplementary materials. It is essential to adjust these factors to achieve the desired workability for specific construction requirements.

- Other Slump Ranges:
It's worth mentioning that different slump ranges correspond to different degrees of workability. For example, a slump value below 25 mm indicates low workability, while a slump value above 175 mm suggests very high workability or excessive fluidity.

In conclusion, the degree of workability is considered medium when the value of slump in mm falls within the range of 50 – 100. This range ensures that the concrete mixture has a balanced flowability and cohesiveness, making it suitable for a wide range of construction applications.