Which of the following is exempt from nematode certification?

Garlic is exempt from nematode certification due to its unique biological properties and the specific impact of nematodes on different types of vegetables. Nematodes are microscopic worms that can be harmful to various crops, but garlic, in particular, has been found to be less susceptible to nematode infestations. This exemption is based on research indicating that garlic does not serve as a suitable host for certain nematodes that are generally a concern for agricultural producers.

In comparison, the other options such as tomatoes, bell peppers, and lettuce can be impacted by nematodes and are often subject to certification requirements to ensure they are free from these pests before being sold or transported. These plants can provide favorable environments for nematodes, which can lead to significant agricultural losses if not properly managed. Hence, the requirement for certification is essential for maintaining agricultural health and preventing the spread of nematode infestations in crops where they pose a threat.