Let's analyze each statement: 1. **The in-plane maximum and minimum normal stresses = The in-plane principal stresses** - **Correct.** The principal stresses in a plane are, by definition, the maximum and minimum normal stresses at a point in the plane. 2. **All** - This implies all the statements are correct, but let's check the rest. 3. **The in-plane principal stresses, act on the principal planes of stress** - **Correct.** Principal stresses act on principal planes, by definition. 4. **No shear stress acts on the principal planes** - **Correct.** Principal planes are oriented such that the shear stress is zero on them. --- **Conclusion:** **All** the statements are correct. So, the correct answer is: **All**