$b)$. The biuret test, also known as Piotrowski's test, is a chemical test used for detecting the presence of peptide bonds. In the presence of peptides, a copper $(II)$ ion forms mauve-colored coordination complexes in an alkaline solution.

$c)$.Fehling’s test is used for reducing sugars and non-reducing sugars, supplementary to the Tollens’ reagent test. The test was developed by German chemist Hermann von Fehling in $1849$.

$d).$ Millon's reagent is an analytical reagent used to detect the presence of soluble proteins.