This tells us that the symptoms experienced by these workers are likely caused by MBTE and not by something else that would affect all oil-refinery workers, like the gasoline itself. By ruling out an alternate explanation, this choice strengthens the toxicologist's argument.

This doesn't strengthen the argument. Knowing about other medical conditions that can lead to the same symptoms doesn't help us determine whether those symptoms will increase because of gasoline containing MBTE.

This tells us that areas that use gasoline containing MBTE have already seen an increase in headaches, fatigue, and shortness of breath in ordinary people. This strengthens the author's argument that increased use of MBTE-containing gasoline will cause increased rates of those symptoms overall.

Compared with MBTE-free gasoline, gasoline containing MBTE is positively correlated with headaches, fatigue, and shortness of breath. This strengthens the toxicologist's argument.

This strengthens the toxicologist's argument, because it tells us that the survey was representative. The refinery workers in the survey weren't unusually predisposed to these symptoms for some other reason.