This is the opposite of what we need. If most of the nearby vacant lakes are already proven to be suitable breading territories, it would make sense for the loons to settle in those lakes rather than fighting over the occupied ones!

This doesn’t matter. We want to know why these contests are taking place at all, not who starts them—why wouldn’t the loons just settle in a territory they wouldn’t have to compete over?

This doesn’t help. It doesn’t matter how often the loons are successful in their attempted takeovers. We just want to know why they’re trying to take the occupied lakes in the first place.

This is the explanation we need! When loons see another breeding pair in a lake, they take that pair’s presence as evidence that the lake is a good place for breeding. Instead of taking their chances with an untested lake, they try to get the lake someone’s already vetted!

This answer choice would be helpful if it told us that the unoccupied lakes lack some of these factors that make lakes suitable for breeding, but as it stands, (E) doesn’t give us any information that would help explain why loons don’t always settle in unoccupied lakes.