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Option II> I> III More stable compound has lesser heat of hydrogenation. Cyclopropene being aromatic is very stable. In structure (I) methyl group is attached at one end of double bond which imparts stability to it. Therefore (I) is most stable. Structure (III) also has extra stability due to aromatic cyclopropene ring, but methyl group is not attached directly to doubly bonded carbon, therefore it is less stable than (I). Least stable is (II) as the double bond is present outside the ring and no alkyl group is attached to impart extra stability to it. So, the order of heat of hydrogenation is reverse of order of stability.