The correct answer is option 'D' - Knurling.

Explanation:
In lathe operations, the process of increasing the diameter of a workpiece is known as knurling. Knurling is a machining operation that is used to create a pattern of roughened or indented areas on the surface of a workpiece. This pattern provides a better grip or a decorative appearance to the workpiece.

Steps involved in knurling:
1. Selection of knurling tool: A knurling tool consists of two knurls, which are mounted on a holder. The knurls have a pattern of straight or diagonal ridges that will be pressed into the workpiece.

2. Mounting the workpiece: The workpiece is securely mounted on the lathe chuck or between centers, depending on the size and shape of the workpiece.

3. Setting up the lathe: The lathe is set to the appropriate speed and feed rate for the material being knurled. The knurling tool is also properly aligned with the workpiece.

4. Engaging the knurling tool: The knurling tool is brought into contact with the rotating workpiece. The knurls on the tool press into the surface of the workpiece, creating the desired pattern.

5. Feed and depth of cut: The feed and depth of cut are adjusted to control the depth and width of the knurling pattern. The feed is the distance the tool moves along the workpiece for each revolution, and the depth of cut is the distance the knurls penetrate into the workpiece.

6. Knurling process: The knurling tool is fed along the length of the workpiece, creating a continuous pattern of ridges. The tool is then retracted, and the workpiece is rotated to the next position for the next pass.

7. Finishing and inspection: After the knurling operation is complete, the workpiece is inspected for the desired knurling pattern. Any necessary adjustments can be made, and the workpiece can be finished or further processed as required.

Overall, knurling is a useful lathe operation that allows for the enlargement of the diameter of a workpiece while providing enhanced grip or aesthetic appeal.