The Stacks project

Proof. Let $\mathcal{K} = \mathop{\mathrm{Ker}}(\mathcal{F} \to \mathcal{G})$. By right exactness of pullbacks we find that $\mathcal{K}_ T \to \mathcal{F}_ T \to \mathcal{G}_ T \to 0$ is an exact sequecnce for all schemes $T$ over $B$. In particular, a quotient of $\mathcal{G}_ T$ determines a quotient of $\mathcal{F}_ T$ and we obtain our transformation of functors $\mathrm{Quot}_{\mathcal{G}/X/B} \to \mathrm{Quot}_{\mathcal{F}/X/B}$. This transformation is a closed immersion by Flatness on Spaces, Lemma 77.8.6. Namely, given an element $\mathcal{F}_ T \to \mathcal{Q}$ of $\mathrm{Quot}_{\mathcal{F}/X/B}(T)$, then we see that the pull back to $T'/T$ is in the image of the transformation if and only if $\mathcal{K}_{T'} \to \mathcal{Q}_{T'}$ is zero. $\square$