Explanation

(b)
Dimethyl oxalate is used for distinguishing primary, secondary and tertiary amines (Hofmann’s method).
The primary ($1{^\circ}$) amine forms the corresponding substituted oxamide which is usually a crystalline solid.
$COOC_{2}H_{5}\ \ \ \ \ H - NHR$ $CONHR$
$|$ $+$ $\rightarrow$ $|$ $+ 2C_{2}H_{5}OH$
$COOC_{2}H_{5}\ \ \ H - NHR$ $CONHR$
The secondary amine forms a diethyl oxamic ester which is generally a liquid.
$\ \ \ COOC_{2}H_{5}\ \ \ \ \$ $CONR_{2}$
$|$ $+ NHR_{2}$ $\rightarrow$ $|$ $+ C_{2}H_{5}OH$
$COOC_{2}H_{5}$ ${COOC}_{2}H_{5}$
The tertiary amine under these conditions does not react at all since it does not contain a replaceable hydrogen atom.