An alkene, on ozonolysis gives formaldehyde and acetaldehyde. The alkene is

Ozonolysis cleaves the double bond and converts each alkene carbon into a carbonyl compound. To get formaldehyde and acetaldehyde, the alkene must split as follows: $$\mathrm{HCHO} + \mathrm{CH_3CHO}$$ This comes from the alkene: $$\mathrm{CH_2{=}CHCH_3}$$ which is propene. **Checking options:** $\mathrm{Ethene}$ gives only formaldehyde, $\mathrm{Butene\text{-}2}$ gives two acetaldehyde molecules, and $\mathrm{Butene\text{-}1}$ gives formaldehyde plus propanal. **So the correct option is $\mathrm{Propene}$.**