Compare the following foursome of forelimbs — a bat's wing, a penguin's flipper, a lizard's leg, a human's arm. On the surface, the (i)___ appendages look quite dissimilar from one
another, and they perform distinct tasks: flying, swimming, darting, purchasing hands-free electronic devices. Yet beneath the (ii)____ lies skeletal (iii)___, for each forelimb houses the same set of four bones: humérus, radius, ulna, and carpal.

A good question which sets traps in the vocabulary:
Blank (i) I couldnt guess the approximate word, but understood its not taking any stance such as common or different - a bit neutral hence "common" doesnt fit the context and "redundant" is out of scope, leaving us with "various"

Here I started with blank (iii), where it was indicating some form of similarity so "motley" - exact opposite, "disorder" - doesnt fit the context/scope, thus "homogenity"

Now coming to blank (ii) - which is opposite in nature to (iii) due to the use of "yet" keyword. So we are looking for difference or some form of heterogenity. Now the trap is within the answer choices. We can safely eliminate "arrangement" as it is a neutral word and doesnt fit the guess we made.
"miscellany" - which means a mixture (which seems fine, but doesnt exactly match our guess)
Coming to "vagaries" - it seems a tempting option, because it can tend to convey a meaning of "various" - however it refers to the the unexpected changes in one's or something's behavior. e.g. vagaries of the stock market

Given the context, we are simply talking about the different mixture ("miscellany") of functions which are quite standard and mapped to (or expected of) the animal and skeletal structure. If despite the structure, the being lets say "bat" suddenly starts walking or swimming - we may say the "vagaries" (or unexpectedness) of skeletal structure -> which doesnt make sense