Hexane-2,5-dione, $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}{}_2\mathrm{COCH}{}_3$, is warmed with dilute aqueous $\mathrm{NaOH}$. The major final product, formed by an intramolecular aldol condensation, is

The enolate from a terminal $\mathrm{CH}{}_3$ (C-1) attacks the carbonyl at C-5, closing a five-membered ring (atoms C-1 to C-5). The resulting beta-hydroxy ketone dehydrates to the conjugated enone. The ring carbonyl carbon is C-2 of the original chain, and the methyl-bearing alkene carbon is the original C-5, so the product is 3-methylcyclopent-2-en-1-one.