Hexane-2,5-dione, when heated with dilute aqueous NaOH, gives an intramolecular aldol condensation product. The product is

In $\mathrm{CH_3CO{-}CH_2CH_2{-}COCH_3}$ the enolate carbon of one methyl (C-1) can attack the carbonyl carbon C-5: ring includes C-1 to C-5, i.e. a five-membered ring. Attack of C-3 on C-5 would give a strained 3-membered ring. Dehydration of the $\beta$-hydroxy ketone gives the conjugated enone with $\mathrm{C=O}$ at C-2 and the double bond $\mathrm{C1=C5}$, the methyl on C-5: 3-methylcyclopent-2-en-1-one (five-membered rings are preferred over strained alternatives).