Premise - Synthetic chemicals that are carcinogenic are used in very small quantities in food vs. Non synthetic chemicals that are widely found in plants and animals
Conclusion - Therefore the rise in cancer rate is not due to synthetic chemicals
We need to weaken this conclusion
(A) the rise in the cancer rate in recent decades is due to increased exposure to nonsynthetic pollutants - If we say the rise is due to non synthetic, it is like a strengthener for this conclusion saying that yes it's not due to synthetic but due to non synthetic. Opposite answer. Eliminate.
(B) the rise in the cancer rate in recent decades is due to something other than increased exposure to carcinogens - By saying it is due to something else, it is implying it is not due to synthetic chemicals. same as A. Eliminate
(C) some synthetic chemical compounds that are not known to be carcinogenic are in other respects toxic - okay synthetic chemicals are not carcinogenic but toxic. But the argument says it's(carcinogenic/toxic) added in tiny quantities and hence not cancerous. Eliminate.
(D) people undergo significantly less exposure to carcinogens that are not synthetic than to those that are synthetic. - Okay I eat 10 items each with small quantities of synthetic carcinogen say 5gms*10 = 50gms. I eat 2 items of non synthetic carcinogen say 15gms*2 = 30 gms. So even if quantities are less my exposure to synthetic carcinogens is higher. hence it can still be the cause of increase in cancer.
(E) people can vary greatly in their susceptibility to cancers caused by nonsynthetic carcinogens - People can vary in their likelihood of getting cancer be it due to synthetic/nonsynthetic does not in anyway mean an increase in rate of cancer. Does not strengthen or weaken. - Eliminate