Ultrasonic machining is a non-traditional machining process that utilizes ultrasonic vibrations of high frequency to carry out material removal. It is a versatile machining technique that finds applications in various industries. The correct answer to the question is option 'D', which states that ultrasonic machining is used in making dies, machining hard carbides, glasses, and stones, as well as in dental applications. Let's explore each of these applications in detail:

a) Making dies:
- Ultrasonic machining is commonly used in the manufacturing of dies. Dies are essential components used in various industries for shaping or forming materials.
- The ultrasonic vibrations generated during the machining process help in removing material from the workpiece, resulting in the desired shape of the die.
- This process offers high precision and accuracy, making it suitable for producing complex die shapes.

b) Machining hard carbides, glasses, and stones:
- Ultrasonic machining is particularly effective in machining hard and brittle materials such as carbides, glasses, and stones.
- These materials are difficult to machine using conventional methods due to their high hardness and brittleness.
- Ultrasonic machining employs the principle of vibration-induced erosion to remove material from these hard substances.
- The high-frequency vibrations of the tool cause the abrasive particles to impact the workpiece, resulting in material removal.

c) Dental applications:
- Ultrasonic machining is widely used in dental applications, such as the fabrication of dental implants, crowns, and bridges.
- The technique allows for precise shaping and finishing of dental restorations, ensuring a proper fit and aesthetic appearance.
- Ultrasonic machining also enables the removal of hard deposits and calculus from teeth, improving oral hygiene.

In summary, ultrasonic machining is a versatile process that finds applications in the production of dies, machining of hard materials like carbides, glasses, and stones, as well as in dental applications. Its ability to provide high precision, accuracy, and efficiency makes it a preferred choice in these industries.