When a question asks you to "account for a finding," the finding itself must be somewhat unexplained, even a paradox. In this case, we know there are two causes for the decrease in the otter population: pollution and habitat destruction. Because legislation has had more to say about pollution, it makes sense that the effects of pollution would go down, meaning that habitat destruction is the bigger problem. But the argument tells us that pollution remains the bigger problem.

Choice (A) doesn't help--the cost of the legislation doesn't tell us about its effectiveness. (B) is irrelevant, as we care only about sea otters. Similarly, (E) is too general.

Choice (C) is correct. If habitat destruction had almost reached its maximum by 1970, it's only reasonable that it didn't increase by a very large percentage between 1970 and 1990. (E) may sound similar, but it is less directly related. Not only does it discuss legislation (not the specific destruction of habitats), but the endpoint is 1960, not 1970, as it is in the question and in (C).