Explanation:

Concrete workability refers to the ease with which concrete can be mixed, placed, and finished. It is an important property that affects the overall quality and strength of the concrete. One of the common methods to measure workability is through the slump test.

The slump test is a simple and widely used test to measure the consistency or workability of fresh concrete. It involves filling a standard slump cone with concrete, compacting it, and then removing the cone. The resulting slump is the difference in height between the top of the cone and the height of the concrete slump.

The given question states that the low workability of concrete conforms to a slump of 25-75 mm. Let's break down the options to understand why option 'A' (25-75 mm) is the correct answer:

Option A: 25-75 mm
- A slump range of 25-75 mm indicates a relatively low workability of concrete. The concrete will have a stiff consistency and will not flow easily.
- This type of concrete is suitable for applications where minimal flow is required, such as in heavily reinforced structures or vertical elements.
- The low slump indicates that the concrete is not very fluid and may require more effort to place and consolidate.
- The lower end of the range (25 mm) signifies a more rigid and less workable concrete, while the upper end (75 mm) indicates a slightly more flowable but still low workability concrete.

Option B: 50-100 mm
- This range indicates a moderate workability of concrete. It is not as low as option A, but still not as fluid and easy to handle as higher slump ranges.
- A slump range of 50-100 mm is suitable for general construction purposes, where some flowability is required but not excessive.
- The lower end of the range (50 mm) suggests a slightly stiffer concrete, while the upper end (100 mm) indicates a more flowable but still moderate workability concrete.

Option C: 75-100 mm
- This range indicates a moderate to high workability of concrete.
- The lower end of the range (75 mm) suggests a relatively flowable concrete, while the upper end (100 mm) indicates a more fluid and easy-to-handle concrete.
- This range is suitable for applications where a higher degree of flow is required, such as in flat slabs or lightly reinforced elements.

Option D: 100-150 mm
- This range indicates a high workability of concrete.
- The lower end of the range (100 mm) suggests a flowable concrete, while the upper end (150 mm) indicates a very fluid and highly workable concrete.
- This range is suitable for applications where a high degree of flow is required, such as in thin sections or congested reinforcement.

In conclusion, the correct answer is option 'A' (25-75 mm) as it represents a low workability of concrete, suitable for applications where minimal flow is required.