The compound which on reaction with aqueous nitrous acid at low temperature produces an oily nitrosoamine is

2° amine produces oily nitrosoamine. When they combine with aqueous ${\mathrm{HNO}}_2$ at a low temperature, secondary amines (NH group) produce an oily nitrosoamine.
Diethylamine is a 2 amine as a result. yields an oily product at a low temperature with ${\mathrm{HNO}}_2$.

Option C is correct.