The plate loading test is a commonly used method to determine the bearing capacity of soils. It involves applying a known load onto a square bearing plate and measuring the settlement or deflection of the plate under the load. This test is typically performed on the ground surface, and the results are used to estimate the safe load-bearing capacity of the soil.

The size of the square bearing plate used in the test is an important factor that affects the accuracy and reliability of the test results. The correct answer to the given question is option 'B', which states that the size of the square bearing plate should be between 300 mm and 750 mm.

Here is an explanation of why this range of plate size is appropriate for the plate loading test:

Importance of Plate Size:
- The size of the bearing plate determines the area over which the load is distributed. A larger plate distributes the load over a larger area, resulting in a lower stress concentration and more representative results.
- On the other hand, a smaller plate concentrates the load over a smaller area, which can lead to localized failure and inaccurate results.

Factors Influencing Plate Size:
1. Soil Type: The bearing capacity of soils varies depending on their type and properties. Different soil types may require different plate sizes to accurately measure their bearing capacity.

2. Test Purpose: The purpose of the plate loading test can also influence the plate size selection. For example, if the test is conducted to estimate the bearing capacity of a specific foundation, the plate size should be similar to the size of the actual foundation.

3. Available Equipment: The equipment used for the test, such as the loading device and measurement instruments, may have limitations on the size of the plate that can be used.

Appropriate Plate Size Range:
- The range of 300 mm to 750 mm for the square bearing plate is considered appropriate for most soil types and testing purposes.
- Plates smaller than 300 mm may result in localized failure and inaccurate results, especially for cohesive soils or weak soils.
- Plates larger than 750 mm may become cumbersome to handle and may exceed the capacity of the available testing equipment.

Conclusion:
In summary, the size of the square bearing plate used in the plate loading test for determining the bearing capacity of soils should be between 300 mm and 750 mm. This range provides a good balance between load distribution, accuracy, and practicality for most soil types and testing purposes.