Buttress threads are a type of screw thread that combines the features of both square and trapezoidal threads. They are primarily used for applications that require high axial thrust in one direction, making them suitable for devices such as vices, presses, and jacks. Here is a detailed explanation of each statement:

a) Combination of square and trapezoidal threads:
- A buttress thread is a type of asymmetrical thread where one side has a steep angle resembling a square thread, while the other side has a shallow angle similar to a trapezoidal thread.
- The steep side of the thread is called the load flank, and it is designed to handle axial loads and provide high strength.
- The shallow side is called the lead flank, and its purpose is to provide easy engagement and disengagement of the thread.

b) Transmit motion in one direction only:
- Buttress threads are designed to transmit motion and axial thrust in one direction only.
- The load flank of the thread is optimized for resisting forces in one direction, providing high efficiency and load-carrying capacity.
- In the opposite direction, the shallow lead flank allows for quick disengagement of the thread, making it unsuitable for transmitting motion in that direction.

c) They are used in vices:
- Vices are mechanical devices used to hold workpieces securely during operations such as cutting, drilling, or milling.
- Buttress threads are often used in the design of vices because they can handle high axial loads and provide a strong clamping force.
- The asymmetrical nature of buttress threads ensures that the vice remains securely locked in place, preventing any slippage or movement during operation.

d) All of the mentioned:
- The correct answer is option 'D' because all of the mentioned statements are true for buttress threads.
- Buttress threads are a combination of square and trapezoidal threads, they transmit motion in one direction only, and they are commonly used in vices.

In conclusion, buttress threads are a unique type of screw thread that combines the features of square and trapezoidal threads. They are suitable for applications requiring high axial thrust in one direction, such as vices.