Definition of Diameter of the Cylindrical Rod
The diameter of the cylindrical rod on which threads are cut refers to the measurement of the rod's width. It is an important parameter in thread cutting processes as it determines the size and fit of the threaded connection.

Types of Diameters in Thread Cutting
In thread cutting processes, several diameters are considered. These include:

1. Nominal Diameter:
- The nominal diameter is the theoretical or designated value assigned to a thread size.
- It represents the approximate diameter of the rod or the major diameter of the external thread.
- The nominal diameter is commonly used to specify the size of threaded fasteners.

2. Major Diameter:
- The major diameter, also known as the outside diameter, is the largest diameter of the external thread.
- In other words, it represents the maximum width of the thread.
- The major diameter is measured across the crests of the thread and is an important consideration in determining the fit and compatibility of threaded connections.

3. Root Diameter:
- The root diameter is the smallest diameter of the external thread.
- It is measured between the roots or valleys of the thread profile.
- The root diameter is crucial in determining the strength and load-bearing capacity of the threaded connection.

4. Pitch Diameter:
- The pitch diameter is the diameter at which the width of the thread is exactly half the width of the major diameter.
- It is the effective diameter of the thread and is used to determine the pitch of the thread.
- The pitch diameter is calculated based on the number of threads per unit length and the thread profile.

Explanation of the Correct Answer
The correct answer to the question is option 'A' - Nominal Diameter. This is because the nominal diameter is the designated size of the thread, which can be equivalent to the major diameter of the external thread. The nominal diameter is commonly used to specify the size of threaded fasteners and is an essential parameter in thread cutting processes.

While the other diameters (major, root, and pitch) are also important in thread cutting, they do not specifically refer to the diameter of the cylindrical rod on which threads are cut. Instead, they represent different aspects of the thread profile, such as the maximum width (major diameter), the minimum width (root diameter), and the effective diameter (pitch diameter).

Therefore, the diameter of the cylindrical rod on which threads are cut is known as the nominal diameter. It serves as a reference for the size and fit of threaded connections and is crucial in ensuring proper thread engagement and functionality.