here's another matrix which I don't understand:
What's the solution and why?
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The answer is
The top right option
There are 3 types of shapes. The circle (top center), triangle (center), and square with the top missing (middle right). There are two other variations of these. One where the left and right half is swapped (middle left, top right, and top left), and one where the right half is rotated 180 degrees (bottom left and bottom center). We are missing the second variation of the square.