Official explanation from MGRE:
If the area of square ACBD is 9, then its sides each have length 3. If the area of square DEFG is 16, then its sides each have length 4. Specifically, two of the sides of the triangle (BD and DE) have lengths 3 and 4, respectively.
The area of a triangle is ½ bh, and is the same regardless of which side is used as the base. So, use DE = 4 as the base of the triangle. If this side is the base, then the height is as shown:
Using side DE as the base, the height must be no greater than 3, since it cannot be longer than the sides of the triangle. (The greatest possible value for the height is exactly 3, which will only be the case if the triangle is a right triangle.) Therefore, the area must be no greater than ½(3)(4) = 6. So, Quantity B is always greater.
The correct answer is B.