Hi MarceloBR,
Brent's posted a full explanation for this question, so I won't rehash it here. Instead, I want to point out the importance of certain "math" ideas that you'll find useful on the GMAT:
1) Any diagonal line segment on a graph forms the hypotenuse of a right triangle. You can figure out the base and height of this right triangle, so you can figure out the length of the diagonal line.
2) GMAT questions are often based on patterns, so there are rarely any co-incidences in the numbers that appear, the wording of the question or how the answer choices are designed. Here, you should recognize that root3 and 1 are numbers that appear in a very specific type of right triangle. Those numbers are NOT by accident; they're there to test your ability to spot (and take advantage of) a math pattern.
GMAT assassins aren't born, they're made,
Rich