**Explanation:**

To understand why the correct answer is option B, let's first define the terms mentioned in the question:

1. **Nominal diameter:** It refers to the basic or theoretical diameter of a bolt. It is the diameter specified in the bolt size designation.

2. **Minor diameter:** It is the smallest diameter of a screw thread measured at the bottom of the threads.

3. **Major diameter:** It is the largest diameter of a screw thread measured at the top of the threads.

Now, let's discuss why the shank diameter of a bolt is equal to the minor diameter and not the other options:

**1. Nominal diameter:**
The nominal diameter is the theoretical diameter of the bolt, which refers to the size of the bolt specified in its designation. However, the shank diameter is not necessarily equal to the nominal diameter. In fact, the shank diameter is generally smaller than the nominal diameter because the threads are cut into the shank, reducing its diameter.

**2. Major diameter:**
The major diameter is the largest diameter of the screw thread, measured at the top of the threads. The shank diameter is not equal to the major diameter because the shank is not threaded. The major diameter is only applicable to the threaded portion of the bolt.

**3. Minor diameter:**
The minor diameter is the smallest diameter of the screw thread, measured at the bottom of the threads. The shank diameter of a bolt is equal to the minor diameter because it is the diameter of the unthreaded portion of the bolt. The shank is smooth and has a constant diameter throughout its length, which matches the minor diameter of the screw thread.

Therefore, the correct answer is option B, as the shank diameter of a bolt is equal to the minor diameter.