Not unlike many birds' nests, the Musgum people of Cameroon build huts

Regarding B vs. D, there are a couple cool lessons involved in that decision:

1) While students tend to hate the "double negative" feeling of the phrase "not unlike," it's a valid phrase that has been used in correct answers on the GMAT. Temurkhon and gmatime nailed the meaning here: I always look at "not unlike" as meaning "more similar than you might think" or "different, but with enough similarities that the comparison deserves attention." So it's helpful to just be familiar with that phrase in case it comes up: it's a valid structure.

2) Way more likely to be problematic than "not unlike" is "just like." "Not unlike" as you see above is pretty hard to logically disprove, as it encompasses both "it's different" but "there are similarities." (It's like a weatherman saying "chance of rain" - he's right either way whether it rains or doesn't). But "just like" has a really high bar to hit - it's essentially saying "with regard to what's being discussed, these two things are exactly the same." And here, the non-underlined portion includes a difference that violates "just like" - when it says that the Musgum people "use few, if any, sophisticated tools" it's showing a big difference between humans and birds. Birds just do not and cannot use sophisticated tools. The comparison being drawn here is about the method in which the two types of dwellings - nests and huts - are made, and the inclusion of sophisticated tools for the human huts means that those methods are decidedly different.

3) Larger than the precision in language of comparison diction - the GMAT has evolved toward more and more logical meaning decisions in Sentence Correction. So train yourself to see those kinds of logical differences. The testmaker is pretty good at hiding them farther from the underline than many examinees are prepared to go - had this sentence ended at "are made mostly of mud, thatch, and water" B is a very viable choice. "Just like" works well there. But when you add "using few, if any, sophisticated tools" - which seems at first glance to just be filler text to round out the sentence - the logic changes. Especially on harder questions, the whole sentence tends to matter: if you're struggling to make a decision between 2 or 3 final answer choices, the important element may well lie far from the underlined portion.