The dehydration of malonic acid $CH_2(COOH)_2$ with $P_4O_{10}$ gives:

When formic acid is heated with conc. $H_{2}SO_{4}$,the gas evolved is

Most acidic among the given acids is:

Formaldehyde and formic acid can be distinguished using

$n$-Propanol on treatment with concentrated $HBr$ gives $P$. The product $P$ on reaction with $KCN$ gave the product $Q$. The product $Q$ on heating with aqueous acidic solution,furnished the product $R$. The product $R$ is

What is the correct order of acidity of compounds $(I)$,$(II)$,and $(III)$?
$(I)$ $4$-Nitrobenzoic acid
$(II)$ $4$-Methoxybenzoic acid
$(III)$ Benzoic acid