What is the correct order of acidic strength?

Which of the following compounds is most reactive with $NaHCO_3$?

The major products obtained in the reaction of oxalic acid with conc. $H_2SO_4$ upon heating are

Arrange the products $I$,$II$,$III$ from the following reactions in decreasing order of their acid strength.
$A) \text{Propylbenzene} \xrightarrow[\text{ (ii) } H_3O^{+}]{\text{ (i) } KMnO_4 / OH^{-}} I$
$B) CH_3COOH \xrightarrow[\text{ (ii) } H_2O]{\text{ (i) } Br_2 / \text{red } P} II$
$C) \text{Bromobenzene}$ $\xrightarrow[\substack{\text{ (ii) } CO_2 / \text{dry ether} \\ \text{ (iii) } H_3O^{+}}]{\text{ (i) } Mg / \text{dry ether}} III$

What is the product obtained when acetamide is treated with nitrous acid $(HNO_2)$?

Which of the following has maximum reactivity towards nucleophilic attack?