The question asks us to select the compound with the smallest heat of hydrogenation per mole out of the following:
Since all four compounds form the same product (butane), a comparison without actual data should be possible. 1,3-butadiene is conjugated so it is the most stable compound and therefore its heat of hydrogenation should be the smallest.
But it requires 2 moles of hydrogen for complete conversion to butane. And the answer is actually trans-2-butene.
So would we consider each heat of hydrogenation "per mole of hydrogen" or "per mole of the given compound"? The question isn't clear about this and also, if we do consider it as "per mole of hydrogen" then is it correct to simply half the value heat of hydrogenation for 2 moles?