Which of the following compounds gives a positive test with $2,4$-$DNP$ ($2$,$4$-dinitrophenylhydrazine)?
$(a)$ Acetone
$(b)$ Butan-$2$-one
$(c)$ Acetaldehyde
$(d)$ Formaldehyde

Which of the following aldehydes is most reactive towards nucleophilic addition reactions?

Acetaldehyde reacts with $C_2H_5MgCl$. The final product is

Identify $X$ and $Y$ in the following reaction:

The products of acid hydrolysis of $P$ and $Q$ can be distinguished by:
$P = H_2C=C(CH_3)(OCOCH_3)$
$Q = H_3C-CH=CH(OCOCH_3)$

What are $A, B, C$ and $D$ in the following cross-aldol condensation reaction?
$\text{Propanal} + \text{Ethanal} \xrightarrow[ii) \ \Delta]{i) \ NaOH} A + B + C + D$
(The products are the four possible $\alpha,\beta$-unsaturated aldehydes formed by self and cross-aldol condensation followed by dehydration.)