This is a tricky question in so far as whether it tests pronoun ambiguity or //ism. However, together with these two factors, in my personal opinion, there is a third but IMO primary factor, namely the use of appropriates a subordinate conjunction. The text intends to say that it afflicts some people at the same time it spares some others. This simultaneity is the essence of this topic, I feel.
(A) though it spares… The pronoun’s ambiguity, though not very critical, is in play and the conjunction – though - shows contrast but not concurrentness
(B) where it spares… The pronoun’s ambiguity, though not very critical is in play and the conjunction – where - shows a place of action rather than concurrentness
(C) despite sparing ------ Despite does not bring out simultaneity
(D) when sparing ------ When does not bring out contrast
(E) while sparing -------- while brings out contrast as well as simultaneity; hence, the preferred choice.
However, do GMAT questions skate on such thin ice? Very odd.
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