Cyclohexanone is treated successively with (i) CH3MgBr in dry ether followed by aqueous NH4Cl; (ii) conc. H2SO4, heat; (iii) O3 at 195 K followed by…

Step (i): Grignard addition gives 1-methylcyclohexanol. Step (ii): acid-catalysed dehydration follows Saytzeff's rule via the tertiary carbocation and gives 1-methylcyclohexene (endocyclic alkene). Step (iii): reductive ozonolysis cleaves the ring C=C and gives the open-chain keto-aldehyde 6-oxoheptanal, $\mathrm{OHC(CH_2)_4COCH_3}$, $\mathrm{C_7H_{12}O_2}$. Step (iv): in base, the ketone forms an enolate. Attack through the ketone's ring-side $\alpha$-carbon (C-5) on the aldehyde C-1 closes a five-membered ring (C1 to C5), favoured over the seven-membered ring that the $\mathrm{CH_3}$ enolate would form. Dehydration of the aldol gives the conjugated enone 1-acetylcyclopentene, $\mathrm{C_7H_{10}O}$. The other options are not possible products: 2-methylcyclohept-2-en-1-one is $\mathrm{C_8H_{12}O}$ (one carbon too many; the seven-membered closure through the $\mathrm{CH_3}$ enolate would give plain cyclohept-2-en-1-one), and cyclohex-1-enecarbaldehyde and 3-methylcyclohex-2-en-1-one contain six-membered rings, which no enolate of 6-oxoheptanal can form (the available closures give five- or seven-membered rings only).