(E) says, "Unlike spruces or firs, most sugar maples are native to areas that receive a lot of acid rain."
We're looking to resolve the paradox in the stimulus. The stimulus makes a comparison between maples in acid-drenched, calcium-poor forests with similarly situated spruces and firs. The problem with (E) is that it doesn't tell us why a maple and fir, when placed in the same poor situation, respond so vastly different to their adversity; rather, it simply tells us that a maple is more likely to find itself in that type of bad situation. Thus, (E) is out.
(C) says, "Spruces and firs, unlike sugar maples, can extract calcium from a mineral compound that is common in soil and is not affected by acid rain."
(C) starts off by comparing spruces and firs with sugar maples. It provides us with just what we need by giving us a way in which sugar maples are at a disadvantage in dealing with low calcium levels, helping to explain the greater decline among sugar maples. Thus, (C) resolves the paradox and is the correct answer choice.
Does this make sense? Let us know if you have any other questions!