Determining the ultimate bearing capacity of soil is an essential step in designing foundations and structures. One commonly used method to determine this is the plate load test, which involves applying a load to a square bearing plate and measuring the settlement of the soil. The size and thickness of the bearing plate used in this test are crucial factors that can affect the accuracy of the results.

The recommended size of the square bearing plate used in a plate load test is between 30 and 75 cm. This range allows for a sufficient area of contact with the soil, ensuring that the load is distributed evenly. A larger plate size helps to minimize the effects of local soil variations and provides a more representative measurement of the soil's bearing capacity.

The minimum thickness of the bearing plate is not specified in the given options. However, it is important to have a minimum thickness to ensure that the plate does not deform significantly under the applied load. A thicker plate can distribute the load over a larger area, minimizing the settlement and ensuring accurate results. Therefore, the correct answer is "None of the above" (option D), as the minimum thickness is not mentioned in the given options.

In practice, the thickness of the bearing plate used in a plate load test would typically be determined based on the expected magnitude of the applied load and the properties of the soil being tested. Factors such as the depth of the test, the type of soil, and the expected settlement also play a role in determining the appropriate thickness.

It is important to note that the plate load test is just one method of determining the ultimate bearing capacity of soil. Other methods, such as the standard penetration test (SPT) and cone penetration test (CPT), may also be used depending on the specific requirements and conditions of the project. Consulting with a geotechnical engineer or soil testing specialist is recommended to determine the most suitable testing method and equipment for a given project.