Introduction:
In concrete mix design, various factors are considered to ensure the desired properties and performance of concrete. The correct answer to the given question is option 'D' – Initial setting time of cement. This means that the initial setting time of cement is not a requirement in concrete mix design.

Explanation:
Concrete mix design involves determining the proportions of various ingredients in concrete to achieve the desired properties such as workability, strength, durability, and others. The main requirements in concrete mix design are as follows:

1. Degree of quality control of concrete:
- The degree of quality control refers to the level of monitoring and inspection required during the production and placement of concrete.
- It ensures that the concrete is of consistent quality and meets the specified standards and requirements.
- The degree of quality control can vary depending on the project specifications and the importance of the structure.

2. Workability of concrete:
- Workability refers to the ease and ability of freshly mixed concrete to be placed, compacted, and finished without segregation or bleeding.
- It is an important property as it affects the ease of construction and the final appearance of the concrete.
- Workability can be influenced by factors such as water-cement ratio, aggregate properties, and admixtures.

3. Characteristic compressive strength of concrete at 28 days:
- The characteristic compressive strength is the strength value below which a given proportion of test results falls.
- It is specified at the age of 28 days as concrete gains strength over time.
- The characteristic compressive strength is an important requirement as it determines the load-carrying capacity of the structure and ensures its structural integrity.

4. Initial setting time of cement:
- The initial setting time of cement is the time taken for the cement paste to lose its plasticity and become stiff enough to resist light pressure.
- It is an important property of cement, but it is not directly considered in concrete mix design.
- The initial setting time can be influenced by factors such as cement type, water content, temperature, and admixtures.
- While it is important for the overall construction process, it does not directly affect the proportions of other ingredients in concrete mix design.

Conclusion:
In concrete mix design, the requirements include the degree of quality control, workability, and characteristic compressive strength of concrete. The initial setting time of cement, while important for the construction process, is not a requirement in concrete mix design.