Which alkene on ozonolysis gives $CH_3CH_2CHO$ and $CH_3COCH_3$ : -

In ozonolysis, the C=C double bond is cleaved and each carbon of the double bond becomes a C=O group.
To find the alkene, remove the oxygen atoms from the two carbonyl compounds and join the two carbonyl carbons by a double bond.
$CH_3CH_2CH=O$ + $O=C(CH_3)_2$ $\Rightarrow$ $CH_3CH_2CH=C(CH_3)_2$ (2-methylpent-2-ene)
Check: $CH_3CH_2CH=C(CH_3)_2 \xrightarrow{(i)\ O_3\ (ii)\ Zn/H_2O} CH_3CH_2CHO + CH_3COCH_3$
So the alkene is $CH_3CH_2CH=C(CH_3)_2$.