How is the purity of a compound determined?

During Lassaigne's test,if $N$ and $S$ are present individually in an organic compound,they are converted into:

Which of the following methods is used to check the purity of an organic compound?

Identify the binary mixture$(s)$ that can be separated into individual compounds by differential extraction,as shown in the given scheme.
$(A)$ $C_6H_5OH$ and $C_6H_5COOH$
$(B)$ $C_6H_5COOH$ and $C_6H_5CH_2OH$
$(C)$ $C_6H_5CH_2OH$ and $C_6H_5OH$
$(D)$ $C_6H_5CH_2OH$ and $C_6H_5CH_2COOH$

The Kjeldahl's method for the estimation of nitrogen can be used to estimate the amount of nitrogen in which one of the following compounds?

In Lassaigne's extract of an organic compound,both nitrogen and sulphur are present,which gives blood red colour with $Fe^{3+}$ due to the formation of-