Option 1: Lessen use of pesticides. Crop rotation can contribute to reducing the need for pesticides because rotating crops can disrupt the life cycles of pests and limit their ability to establish and spread within a specific crop. However, option 1 is not the main purpose of crop rotation.

Option 2: Eliminate parasites which have selective hosts. This is the correct answer. Crop rotation involves systematically changing the types of crops grown in a specific area to break the life cycles of parasites that target specific crops. By rotating crops, parasites that rely on a particular host plant are unable to survive and reproduce, reducing the risk of infestation.

Option 3: Yield more crops. Although crop rotation can improve yields indirectly by improving soil health and preventing depletion of nutrients, this is not the primary goal of crop rotation.

Option 4: Produce a greater choice of plant products. While it is true that crop rotation allows for the cultivation of a diverse range of crops, this is not the primary purpose of crop rotation. Its main goal is to manage pests and ensure long-term soil fertility.

In conclusion, crop rotation helps to eliminate parasites that have selective hosts by disrupting their life cycles.