The columnist's conclusion is: "In addition to jeopardizing the health of people in these other countries, this practice greatly increases the health risk to U.S. consumers."
Why? We are told that these "pesticides are often used on agricultural products imported into the United States." And we know that these pesticides, though are banned for use in the United States for nearly 30 years, are still manufactured in the U.S. and exported to other countries.
So, remember that our conclusion is that the United States' practice of continuing to manufacture these banned pesticides and exporting them to other countries not only jeopardizes the health of people in the other countries, but also GREATLY INCREASES the health risk to U.S. consumers.
But, if there were other countries that manufactured and exported these same pesticides, then the U.S.'s participation in the same actions would not necessarily GREATLY INCREASE the health risk to U.S. consumers, rather it might merely be a contributing factor.
This is exactly what answer choice (C) is getting at: "The United States is not the only country that manufactures and exports the pesticides that are banned for use in the United States."
If there are other countries doing the same thing, then the U.S. is merely complicit, rather than being the reason for a huge increase in risk to U.S. consumers.
Hope that clears things up! Please let us know if you have any other questions.
