Step 1: Find the premises and the conclusion. Here's a rough summary of the argument.

Premise: This year a lot of Robins in Northfield have died from infectious diseases.
Premise: Most of the dead Robins had compounds in them that hurt their immunity.
Premise: The only local source of these compounds is the Northfield Chemical Plant.
Conclusion: If the plant stops making the compounds, then the death rate of Robins in Northfield will plummet.

Step 2: Choose the answer that best supports the conclusion. First, think about how to cast doubt on the conclusion. The conclusion here is an "if-then" or conditional statement. Challenge it by explaining how the "if" part could be true yet the "then" part still false.

Imagine that the Northfield Chemical Plant stops producing the compounds, yet the local Robins continue to die in droves from infectious diseases, even months after production has stopped. Why would that happen? Here's an explanation: The compounds remain in the Robin's environment indefinitely unless humans conduct a cleanup. If this statement were true, it would hurt the argument. Is there an answer choice that rebuts it and thus helps the argument? Yes--option (C).

The other options either support the conclusion less than (C) or not at all. For instance, (B) might bolster the link between the compounds and the Robins' reduced immunity by suggesting that those substances correlate with ill-effects in birds. But here's the difference between (B) and (C). If (B) were false, the argument wouldn't really suffer. But it would if (C) were false.