Which of the following compounds will most readily be dehydrated to...

3-Hydroxypentan-2-one and 4-Hydroxypentan-2-one will most readily be dehydrated to give alkene under acidic condition. This will lead to formation of α−β unsaturated ketone (conjugated system). But out of both 4-Hydroxypentan-2-one will most readily dehydrate to give alkene under acidic condition since the carbocation formed will be more stable than in the case of 3-Hydroxypentan-2-one.

Dehydration of 2-Hydroxycyclopentanone will lead to C=C double bond in 5 member ring that will cause strain and product is less likely to be formed.