Which of the following reactions is the decarboxylation reaction?

In the Krebs cycle, oxalosuccinic acid is a 6-carbon compound that undergoes decarboxylation to form $\alpha$-ketoglutarate, which is a 5-carbon compound, releasing one molecule of $CO_2$.

Succinic acid $\rightarrow$ Fumaric acid is an oxidation (dehydrogenation) reaction where $FAD$ is reduced to $FADH_2$.

Cis-aconitate $\rightarrow$ Isocitric acid is a hydration reaction involving the addition of a water molecule.

Isocitrate $\rightarrow$ Oxalosuccinic acid is an oxidation (dehydrogenation) reaction where $NAD^+$ is reduced to $NADH + H^+$.

Therefore, the decarboxylation reaction is Oxalosuccinic acid $\rightarrow$ $\alpha$-Ketoglutarate.

Answer: Oxalosuccinic acid $\rightarrow$ $\alpha$-Ketoglutarate