Which does not react with Fehling solution?

The given compound can be converted into the product by which of the following reagents?

Give applications of cyanohydrins with acetaldehyde with suitable reactions. $OR$ Give following conversions by using acetaldehyde as starting material.
$(a)$ Lactic acid and acrylic acid
$(b)$ Alanine
$(c)$ $1$-amino propane-$2$-ol

The final product $(C)$ in the given reaction sequence is
$C_6H_5COOH$ $\xrightarrow{SOCl_2} (A)$ $\xrightarrow[\text{anhy. } AlCl_3]{C_6H_6} (B)$ $\xrightarrow[\text{(ii) } KOH / (CH_2OH)_2]{\text{(i) } NH_2-NH_2} (C)$

The reaction of $CH_3CH_2CHO$ with $PCl_5$ yields a product. Identify the type of dihalide formed.

$4$-formylbenzoic acid on treatment with one equivalent of hydrazine followed by heating with alc. $KOH$ gives the major product: