reto wrote:

Is quadrilateral ABCD a square?

(1) ABCD is a rectangle whose diagonals intersect at right angles.

(2) AC, a diagonal of ABCD, is also the hypotenuse of isosceles right triangles ABC and ADC.

I am always struggling with such questions. Any sources to prepare for those?

reto, look at the attached picture of quadrilateral family in GMAT.

Statement 2, is sufficient to answer as it can only be a square. Kite would have been possible had the statement mentioned that the triangles are isosceles but not necessarily right angled.

Statement 1 eliminates rhombus as it is not a rectangle.

Thus D is the correct answer.