Answer:
b. How to sell a product is the correct answer.
Explanation:
The goal of the descriptive speech is to narrate about an event, phenomenon or an object so that the listener can visualize the image being portrayed by the speaker.
How to sell a product is the right topic for descriptive speech as the speaker will have to describe about the object and its qualities. Later he will have to describe how it can be sold.
So option b is the correct topic for descriptive speech