This is impossible, please help.
I believe it is
The top two rows each contain one box with a single squiggle, one with two squiggles, and one with three squiggles. The bottom row does not yet contain a box with only one squiggle, so it must be either D, E, or F.
Furthermore, there are three boxes with a square and three with a diamond, but only two boxes with a triangle and two with a circle. D is the only box with both a triangle and a circle - putting it in the empty space would result in the grid having three of every shape.