The compound has conjugated double bonds. Hydrogenation occurs at the double bond with the lowest activation energy, which is typically the most electron-deficient one due to adjacent electron-withdrawing groups.

In this structure, the carbonyl group (C=O) is a strong electron-withdrawing group. Double bond 'c' is conjugated and directly adjacent to the carbonyl, making it the most electron-deficient and thus the most reactive towards hydrogenation. The reaction follows Markovnikov's rule for selectivity.

Final answer: $c$