The correct answer is option 'B': Addendum.

Explanation:
The radial distance from the pitch circle to the top of the tooth is known as the addendum. In gear terminology, the addendum refers to the height of the tooth above the pitch circle.

To understand this concept, let's first define a few terms related to gears:

1. Pitch Circle: The pitch circle is an imaginary circle that represents the theoretical contact between two meshing gears. It is determined by the number of teeth and the diametral pitch of the gear.

2. Addendum: The addendum is the radial distance from the pitch circle to the top of the tooth. It is usually equal to the module of the gear.

3. Dedendum: The dedendum is the radial distance from the pitch circle to the bottom of the tooth. It is usually equal to 1.25 times the module of the gear.

4. Module: The module is a measurement used to determine the size of a gear. It is the ratio of the pitch diameter to the number of teeth. The module defines the size of the gear teeth and is typically measured in millimeters.

Now, let's focus on the addendum. The addendum is an important parameter in gear design because it determines the strength and durability of the gear tooth. A larger addendum results in a taller tooth, which increases the contact area between the gears and improves load distribution. This helps to reduce stress and wear on the teeth, resulting in a more reliable and efficient gear system.

On the other hand, a smaller addendum reduces the height of the tooth, which can lead to higher contact stresses and increased risk of tooth failure. Therefore, the addendum is carefully calculated based on the desired gear performance and application requirements.

In summary, the radial distance from the pitch circle to the top of the tooth is known as the addendum. It plays a crucial role in determining the strength, durability, and performance of gear teeth.