= Solution
Use the trapezium's printed axes, with principal axis $z$ in its plane and $y$ perpendicular to the plane. For either site pair $(1,2)$ or $(3,4)$, the <group representation> has <characters of a representation> $(2,0,2,0)$ in the order $(E,C_2(z),\sigma(xz),\sigma(yz))$: identity and the molecular-plane mirror fix both <s orbitals>, while the other two operations exchange them. The <character inner products> give \b[$\Gamma_{12}=\Gamma_{34}=A_1\oplus B_1$] in the <C2v molecular point group>.
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