Horses have a unique system of interlocking ligaments and bones in their legs, serving as a kind of sling in which, with their body weight suspended and the muscles in their limbs completely relaxed, the result is they are able to sleep standing up.

a) legs, serving as a kind of sling in which, with their body weight suspended and the muscles in their limbs completely relaxed, the result is
b) legs that serve as a kind of sling in which to suspend their body weight, the muscles in their limbs being completely relaxed; as a result,
c) legs that serve as a kind of sling in which, with their body weight suspended while the muscles in their limbs are completely relaxed, the result is
d) legs that serves as a kind of sling in which, with their body weight suspended and the muscles in their limbs completely relaxed, having the result that
e) legs that serves as a kind of sling in which their body weight is suspended while the muscles in their limbs are completely relaxed; as a result,

OA E

Source: GMAT Prep