Key idea: what does ozonolysis do?

Ozonolysis cleaves a carbon-carbon double bond: $R_1R_2C=CR_3R_4 \xrightarrow[\text{reductive workup}]{O_3} \text{carbonyl compounds}$

Each carbon of the double bond becomes the carbon of a carbonyl group.

When is formaldehyde formed?

Formaldehyde, $HCHO$, is obtained when one carbon of the double bond is a terminal methylene carbon: $R-CH=CH_2$

On ozonolysis, $R-CH=CH_2 \xrightarrow{O_3} R-CHO + HCHO$

So formation of formaldehyde confirms the presence of a terminal alkene unit, i.e. a vinyl group $(-CH=CH_2)$.

Check the options

A: a vinyl group
Yes. A vinyl group contains the terminal double bond fragment $-CH=CH_2$, which gives formaldehyde on ozonolysis.

B: an isopropyl group
Isopropyl group is $-CH(CH_3)_2$. It has no double bond, so ozonolysis is not relevant.

C: an acetylenic triple bond
Ozonolysis of alkynes gives acids or diketones after cleavage, not formaldehyde as the characteristic product here.

D: two ethylenic double bonds
Presence of two double bonds is not specifically confirmed by getting formaldehyde. Formaldehyde specifically indicates a terminal $=CH_2$ group.

Conclusion

The compound must contain a vinyl group.

$\boxed{\text{A}}$