There are some formulas you have to remember for the GRE test. In the geometry section you should have a solid grasp of the course material on Circles, Triangles, Squares and Cubes in a standalone as well as a combination of the shapes.
Coming to the question,
The surface area of a cube is given by the formula $6l^2$; where l = length of a side of the cube.
We are given with the diagonal of one side of the cube. We know that the diagonal and two sides of the cube form a 45-45-90 triangle.
In such a 45-45-90 triangle sides are in the proportion of $x:x:x\sqrt{2}$

From the question:
Now, $6l^2$ = 54
or, $l^2$ = 9
or, $l$ = 3

Hence the two sides of the cube are 3 each and the diagonal is 3$\sqrt{2}$.
Therefore, A is the answer.