woohoo921
I marked Choice E as wrong because I thought there needed to be a has after "but". The wording is awkward, but why do you not need a has? Thank you in advance

This is a fairly common construction. Consider an example:
"Tim's children had 17 temper tantrums two nights ago but only 12 tantrums last night, leading Tim to conclude that the medication he was slipping into his children's pancakes was working."
Because the components in red are both objects of the action, "had," there's no need to repeat the verb. We know the writer must mean that the kids
had 17 tantrums one night and
had 12 tantrums the next night, as there's no other way to interpret the sentence. This construction is perfectly fine.
Same deal in (E). In "Laos has a
land area...but a
population," the parts in red are again objects of the same verb, "has," so it's not essential to repeat it.
The takeaway: Don't try to memorize the idea that objects can be connected with a conjunction! The point here is that if you'd read (E) with an emphasis on meaning, you would have realized that it made perfect sense and held on to it. If an option is logical, and there's a construction that
might be problematic, but you're not 100% sure, try not to use that construction as a decision point! Instead, rely on the issues you're sure about -- concrete grammatical errors and meaning problems.
I hope that helps!