ANS:D - all the above tests.

The triaxial apparatus is commonly used for conducting various types of tests on soil specimens. These tests include:

1. Unconsolidated-Undrained (UU) Test: In this test, the soil specimen is subjected to an axial load while the drainage of water is prevented (undrained condition). The purpose is to evaluate the undrained shear strength and stress-strain behavior of the soil under rapid loading conditions.
2. Consolidated-Undrained (CU) Test: In the CU test, the soil specimen is first subjected to an all-around confining pressure to consolidate it before applying the axial load. During the application of axial load, drainage is prevented (undrained condition). This test helps determine the undrained shear strength of the soil under conditions where consolidation has occurred prior to loading.
3. Drained Test: In drained triaxial tests, drainage of water from the soil specimen is allowed during the application of axial load. This test is used to evaluate the drained shear strength and stress-strain behavior of the soil under conditions where drainage is permitted.

Therefore, the triaxial apparatus is used for conducting all the above tests: unconsolidated-undrained (UU) test, consolidated-undrained (CU) test, and drained test. Each of these tests provides valuable insights into the behavior and characteristics of soils under different loading and drainage conditions.