Concrete is a widely used construction material due to its strength, durability, and workability. The workability of concrete refers to its ability to be easily mixed, placed, and compacted without segregation or bleeding. It is an important property as it affects the ease of construction and the quality of the final product. There are several tests that are commonly used to assess the workability of concrete, including the slump test and the compaction factor test.

1. Slump Test:
- The slump test is a simple and widely used test to measure the workability of concrete.
- It involves filling a slump cone with freshly mixed concrete and then removing the cone to allow the concrete to settle.
- The amount of settlement or "slump" of the concrete is measured and indicates the workability of the concrete.
- A higher slump value indicates higher workability, while a lower value indicates lower workability.
- This test provides a quick and easy way to assess the consistency and flowability of the concrete.

2. Compaction Factor Test:
- The compaction factor test is another method used to evaluate the workability of concrete.
- It measures the degree of compaction achieved by a standard amount of work done on the concrete.
- In this test, a known volume of concrete is placed in a standard mold, compacted using a standard compaction procedure, and then the volume of the compacted concrete is measured.
- The compaction factor is calculated by dividing the original volume by the final compacted volume.
- A higher compaction factor indicates better workability, while a lower value indicates poor workability.
- This test provides a more quantitative measure of the workability of concrete compared to the slump test.

Why option 'D' is correct:
- Both the slump test and the compaction factor test are commonly used methods to assess the workability of concrete.
- The slump test provides a qualitative measure of workability, while the compaction factor test provides a quantitative measure.
- By using both tests, engineers and contractors can get a comprehensive understanding of the workability of the concrete and make necessary adjustments in the mix design or construction process if needed.
- Therefore, option 'D' is the correct answer as it includes both the slump test and the compaction factor test, which are both valid methods for assessing the workability of concrete.