Which round is used for high explosive purposes?

The choice of "All of the above" accurately reflects the fact that each of the rounds listed—M821/A1, M853/A1, and M889—serves high explosive purposes in various roles within military operations.

The M821/A1 is a standard high explosive round that provides effective firepower for engaging enemy personnel and light material, making it a versatile choice for a variety of combat scenarios.

The M853/A1, also known for its high explosive capabilities, is designed to produce significant fragmentation effects, further enhancing its effectiveness against a wide range of targets.

The M889 round, while primarily known for its illumination capabilities, also incorporates high explosive traits that allow it to be utilized in specific offensive operations where light and explosive effects are required.

Each of these rounds is specifically manufactured to fulfill high explosive roles within the mortar systems, indicating that they can all be used effectively for such purposes. Thus, recognizing that all these rounds have high explosive applications leads to the conclusion that "All of the above" is indeed the correct response.