Final answer:
Radon is the least persistent chemical in the environment, while DDT, PCBs, and Agent Orange are examples of persistent organic pollutants (POPs).
Step-by-step explanation:
The chemical that is least persistent in the environment is radon. Radon is a naturally occurring radioactive gas that is released from the ground and can accumulate in homes and buildings. It has a short half-life of about 3.8 days, meaning it quickly decays and does not persist in the environment for a long time. On the other hand, DDT, PCBs, and Agent Orange are all examples of persistent organic pollutants (POPs). These chemicals are resistant to environmental degradation and have been found to bioaccumulate in organisms, leading to long-term effects on ecosystems and human health.