What apparatus is used in the plastic limit test?

The correct apparatus used in the plastic limit test is the glass plate. This test is commonly performed in soil mechanics to determine the plastic limit of a soil sample, which is the moisture content at which the soil changes from a plastic state to a semi-solid state.

In this test, a soil sample is worked on a glass plate, which provides a smooth, non-absorbent surface where the soil can be rolled out to form threads. The glass plate allows for easy observation of the soil's consistency and the ability to manipulate the sample accurately. The threads of soil are rolled until they begin to crumble, indicating that the plastic limit has been reached.

Other options, such as a plastic mold, while potentially useful in different contexts, do not provide the necessary rigid surface required for this specific test. Similarly, a steel ruler and a pneumatic gauge have different functions in materials testing and are not needed for determining the plastic limit. Hence, the glass plate is the quintessential apparatus that ensures accurate and effective execution of the test.