Major minerals are those specified in the first schedule appended in the MMDR Act. There is no official definition for major minerals in the MMDR Act.

1. Bentonite is a minor mineral. It is used in drilling mud, foundry sand, and civil engineering applications.

2. Chromite is a major mineral. It is the only ore of chromium, essential for stainless steel and alloy manufacturing.

3. Kyanite is a major mineral. It is a refractory mineral used in ceramics and high-temperature industries.

4. Sillimanite is a major mineral. It is also a refractory mineral with applications in metallurgy and industrial furnaces.

Therefore, Chromite, Kyanite and Sillimanite are major minerals, whereas Bentonite is a minor mineral.

Therefore, the correct answer is D.