The correct answer doesnt seem very convincing.
While ruling out options, I had actually ruled out the correct option. I feel POE seems is best in this scenario.

(A) People tend to underestimate the risks of swimming in these lakes.
- we dont have any info about the what people like, so irrelevant. (but if we had info/stats that around 5000 people from 8000 swam - I think it would make a really a tempting option)

(B) Respiratory and gastrointestinal illnesses occur at a higher rate as a result of swimming in either of these lakes than they do as a result of swimming in any other lake.
- out of scope, as we dont have info about other lakes

(C) Illnesses of kinds other than respiratory and gastrointestinal are not likely to be associated with swimming in either of these lakes.
- we cant possibly know what other diseases could be caused, OOS

(D) The association between swimming in these lakes and respiratory and gastrointestinal illnesses is evidence of a causal relationship between them.
- as mentioned earlier, I had skipped this thinking only given data may not be sufficient for establishing a causal relationship, BUT if you look at it it covers the exact scope and doesnt bring in any outside information. And moreover we cant rule out the possibility that such data may just be enough for establishing a causal relationship. So among the options, this seems to be a better choice.


(E) A large percentage of the people who swim in these lakes are immune to the diseases that swimming may cause.
- we dont know this from the passage, OOS