Since $G / H$ is a group, there exists an element $aH \in G / H$. By the given condition, every element of $G / H$ is a square, so there exists an element $c \in G$ such that $(aH)^2 = (cH)^2$. This means that $a^2H = c^2H$, which implies that $a^2c^{-2} \in H$.
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