Reductive ozonolysis (O$_3$, then Zn/H$_2$O) of $2$-methylbut-$2$-ene gives:

2-methylbut-2-ene is $\mathrm{(CH_3)_2C=CH-CH_3}$. Ozonolysis cleaves the C=C, giving two carbonyl fragments — one from each side of the original double bond. Left fragment $(CH_3)_2C{=}O$ → **acetone** (a ketone, since that carbon had two methyl groups). Right fragment $CH_3-CH{=}O$ → **acetaldehyde** (an aldehyde, since that carbon had one H). Reductive workup (Zn/H₂O) is what preserves the aldehyde; oxidative workup (H₂O₂) would over-oxidise it to a carboxylic acid.