= Solution
The alternating short and long bonds preserve fourfold, but not eightfold, rotations. Including the molecular plane and <inversion symmetry>, the planar structure belongs to \b[$D_{4h}$], the <D4h molecular point group>.
Use the printed axes: the short bonds are $1$–$2$, $3$–$4$, $5$–$6$ and $7$–$8$, and the $x$ and $y$ axes pass through the midpoints of opposite short bonds. Interchanging short and long bonds is not a <molecular symmetry operation> of this geometry.
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