The optimum rake angle of a tool refers to the angle at which the cutting edge of the tool is positioned relative to the direction of the cutting force. This angle plays a crucial role in determining the efficiency and effectiveness of the cutting process. The correct answer to the given question is option 'C', i.e., properties of work material.

Properties of Work Material:
The properties of the work material being machined have a significant impact on the optimum rake angle of a cutting tool. Some of the key properties that influence the rake angle are:

1. Hardness: Harder materials require a lower rake angle to minimize cutting forces and prevent excessive tool wear. Softer materials, on the other hand, can tolerate higher rake angles for improved chip control and better cutting performance.

2. Toughness: Materials with high toughness, such as stainless steel, require a lower rake angle to reduce cutting forces and prevent tool failure. Brittle materials, like cast iron, can withstand higher rake angles for improved chip control.

3. Ductility: Ductile materials, like aluminum, tend to produce long continuous chips. A moderate rake angle is suitable for efficient chip evacuation. In contrast, materials with low ductility, such as brass, produce short chips that require a higher rake angle for effective chip control.

4. Work Hardening: Some materials, like stainless steel, exhibit work hardening during the cutting process. This means that the material becomes harder and more resistant to cutting as it is deformed. A lower rake angle is preferred to minimize tool wear and cutting forces in such cases.

Other Factors:
While properties of the work material primarily determine the optimum rake angle, it is important to consider other factors as well:

1. Cutting Speed: Higher cutting speeds generally require a lower rake angle to prevent excessive tool wear and maintain cutting efficiency.

2. Cutting Tool Material: Different cutting tool materials may have specific recommendations for the optimum rake angle based on their hardness, toughness, and other characteristics.

3. Feed and Depth of Cut: The feed rate and depth of cut also have an influence on the optimum rake angle. Higher feed rates and deeper cuts may require a lower rake angle to ensure efficient chip evacuation and prevent chip clogging.

In conclusion, while factors such as cutting speed and cutting tool material can have some influence, the properties of the work material being machined play the most significant role in determining the optimum rake angle of a cutting tool. Understanding these properties and selecting the appropriate rake angle is crucial for achieving optimal cutting performance and tool life.