How can it be C? Isn't the sqr root of 1 + -1. In that case, wont the option be D instead of C

GRE Quant Problem Solving
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When the values are under the principal square root then we take positive square root only.

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You are right in actual… because in India we are taught that way…However ETS only considers + value under the root, so only 1/4 will be the answer…You cannot put any negative number under the root even if it becomes +ve with the square.