The outer part of the bottom of cooking utensils is typically left black because black surfaces are known to be excellent absorbers of heat. Black color absorbs a wider spectrum of light and heat energy compared to lighter colors. This characteristic enhances the efficiency of heat transfer from the stove to the utensil, ensuring that the cooking process is faster and more uniform.

The increased heat absorption helps in maintaining a consistent cooking temperature, which is essential for proper cooking. This is particularly beneficial in ensuring that the heat is distributed evenly across the bottom surface of the utensil, reducing cooking times and improving energy efficiency.

While cleanliness and conductivity are important factors, the primary reason for the blackened bottoms is to maximize heat absorption. Therefore, the correct answer is [D] Black surface is a good absorber of heat.