whose can refer to objects as well... who is used only for humans..
further you can eliminate wrong options without caring about whose...
C, D, E.. fall below that of a set minimum... "that of" is useless... making it feel like set minimum has some other level... which is making no sense
A... we are talking of a survey already taken... so continuous tense is not required..
whose
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Source: Beat The GMAT — Sentence Correction |













