Formula for the diagonal of a cube is always:

a\(\sqrt{3}\) where a is the side

Therefore it is just -

= 1\(\sqrt{3}\)

= \(\sqrt{3}\)

Answer is C

Note: Diagonal-focused questions are *very* common on the GMAT both for cubes and cuboids so memorizing this formula can take you a long way. Diagonals of cubes are also sometimes referred to as "longest distance between two points within a cube".
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