Let's analyze the sentence:
"Far from eschewing a(n) $\qquad$ lifestyle and adopting one commensurate with that found in developing nations, the international humanitarian outreach has accorded its members perquisites befitting a decadent monarchy."

The structure contrasts what the organization is not doing with what it is actually doing. It is "far from eschewing" a certain lifestyle and adopting a simpler one (like in developing nations). Instead, it gives its members "perquisites befitting a decadent monarchy" (i.e., lavish benefits).

So, the blank should describe a lifestyle that is opposite to "one commensurate with that found in developing nations" (which implies simplicity, modesty, or austerity). Therefore, the organization is not avoiding a luxurious or extravagant lifestyle; in fact, it is embracing it.

Now, look at the options:
A. noble - implies virtue or high moral qualities, not necessarily luxury.
B. austere - means simple, plain, strict; this is the opposite of what we need because it aligns with "developing nations" lifestyle.
C. conventional - means typical, ordinary; not specifically contrasting with simplicity.
D. extravagant - means luxurious, excessive, lavish; this fits perfectly as it contrasts with the modest lifestyle of developing nations.
E. cloistered - means secluded, sheltered; not directly related to luxury or simplicity.

Thus, "extravagant" is the best choice. The sentence means that instead of avoiding an extravagant lifestyle and adopting a simple one, the organization gives its members perks like those of a decadent monarchy.

Answer: D. extravagant