I am not an instructor but here's why I think C is the correct answer.
The conclusion I identified is "the arrival of humans dramatically decreased the population and diversity of birds on Tiliga."
I think C is the correct answer because (C) "The arrival of a species of microbe, carried by some birds but deadly to many others, immediately preceded the first human immigration to Tiliga" introduces an alternate cause to the effect.
The effect (decrease in the population and diversity of birds on Tiliga) could be due to the deadly species of microbe instead of the arrival of humans thus weakening the conclusion.
