01
Check the vertical and horizontal reflections
Reflecting across the vertical centre line reverses each row. A B A remains A B A, C D C remains C D C, and the last row also remains unchanged. Reflecting across the horizontal centre line swaps the first and third rows, which are identical, while leaving the middle row in place. Both reflections preserve all nine labels. The central D stays fixed in each case. These conclusions follow from the explicit row comparisons, not merely from the rectangular outline, which would remain a square even for many different arrangements of letters.
02
Rotate a quarter-turn and find a specific mismatch
A clockwise quarter-turn produces rows A C A, B D B and A C A. The top-middle cell now contains C, whereas the source top-middle cell contains B. Because B and C are distinct, this one mismatch is enough to reject quarter-turn symmetry. Several other cells also differ, but they are not needed to establish failure. The example therefore has vertical and horizontal reflection symmetry without quarter-turn symmetry. A half-turn does preserve the grid: reversing the row order and reversing every row returns the original three rows.
03
Save the source and the comparison rule together
A reusable sheet records source rows, distinct cell types, the chosen axis or rotation centre, the transformation and the transformed rows. For a passing result, retain the complete comparison; for a failing result, record at least one mismatched position and its before-and-after labels. Here the clockwise quarter-turn witness is row one, column two, changing B to C. If a new exercise treats B and C as the same type, its answer may change because the comparison rule has changed. Preserve that as a separate version instead of overwriting the original result.