1. We need to identify which compound gives acetaldehyde ($CH_3CHO$) on heating with aqueous KOH.

2. Let us check each option.

Option A: $CH_3COCl$

Acetyl chloride undergoes hydrolysis with aqueous KOH: $CH_3COCl + KOH \rightarrow CH_3COOH + KCl$ In alkaline medium, the acid further gives acetate salt. So this does not produce acetaldehyde.

Option B: $CH_3CH_2Cl$

Ethyl chloride with aqueous KOH gives nucleophilic substitution: $CH_3CH_2Cl + KOH \rightarrow CH_3CH_2OH + KCl$ Product is ethanol, not acetaldehyde.

Option C: $CH_2ClCH_2Cl$

1,2-Dichloroethane with aqueous KOH mainly gives ethylene glycol by substitution: $ClCH_2CH_2Cl + 2KOH \rightarrow HOCH_2CH_2OH + 2KCl$ This is not acetaldehyde.

Option D: $CH_3CHCl_2$

This is a gem-dihalide. Gem-dihalides on hydrolysis with aqueous KOH form unstable gem-diols, which lose water to give carbonyl compounds.

First hydrolysis gives: $CH_3CHCl_2 \xrightarrow{aq.KOH} CH_3CH(OH)_2$ The gem-diol is unstable and converts to acetaldehyde: $CH_3CH(OH)_2 \rightarrow CH_3CHO + H_2O$ Hence, this produces acetaldehyde.

3. Therefore, the correct option is: $\boxed{D}$