Hey All,
This is a tough question, so I thought I'd weigh in.
The prompt gives us 2 pieces of information:
PREMISES: Most visitors are not residents of Wyoming, but are residents of the U.S.
That's all we've got. Let's see what we can do with it.
A) most residents of the U.S. are not residents of Wyoming.
Problem: We don't actually know anything about MOST residents of the US from this. This may seem obviously true, but not from the passage. It could just be that some residents of the US are not residents of Wyoming. If that were true, the SOME could still make up most of the visitors to the Park.
B) some residents of Wyoming are not visitors to Yellowstone Park.
Problem: If this were the answer, E would have to work, too. The problem is that, technically, all the residents of Wyoming COULD be going to the park. If a lot more people from other states went, the premises from the original prompt could still be true.
C) most residents of Wyoming are residents of the U.S.
Problem: We don't know much about the residents of Wyoming. Every resident in Wyoming could be a foreign citizen, for all we know. This doesn't relate to the park at all.
D) some residents of the U.S. are not residents of Wyoming.
Answer: We're been told that most visitors to the park are not residents of Wyoming but are residents of the U.S. This means that at least SOME residents of the U.S. are not residents of Wyoming. You can think of it mathematically. Imagine 100 people visited the park. Most of them have to be NOT residents of Wyoming--we'll make that 51--which means 49 of them ARE from Wyoming. We're also told most of them are U.S. residents. This means 51 of them could be U.S. residents, and 49 could be from out of the country. Those out of the country folks can also be part of the NOT residents of Wyoming listed before, of which there are 51. That leaves 2 of them who MUST be U.S. residents that are not residents of Wyoming. Wow. That was epic.
E) some residents of the U.S. are not visitors to Yellowstone Park
Problem: Same as B. Every U.S. citizen could technically visit the park.
Good times. Great oldies. Hope that helped!