Which of the following alkenes forms formaldehyde and acetaldehyde...

Ozonolysis of an alkene involves the cleavage of the carbon-carbon double bond to form carbonyl compounds.

To find the original alkene, we remove the oxygen atoms from the carbonyl products and join the carbon atoms with a double bond.

The given products are formaldehyde (HCHO$) and acetaldehyde (CH_3CHO$).

Joining the carbonyl carbons of $HCHO$ and $CH_3CHO$ gives $CH_3-CH=CH_2$, which is propene.

Reaction:
$CH_3-CH=CH_2 \xrightarrow{O_3, Zn/H_2O} CH_3CHO + HCHO$

Thus, propene forms formaldehyde and acetaldehyde on ozonolysis.

Answer: Propene