Which among the following compounds has the highest boiling point?

Identify compound $A$ used in the following reaction: Benzoic acid $\xrightarrow[\Delta]{A}$ Benzoyl chloride + Phosphorous oxychloride + Hydrogen chloride

What are $A$ and $B$ in the following reaction sequence?
$CH_3COOH$ $\xrightarrow{A} X$ $\xrightarrow[H^{+}]{Y} \underset{\text{(Analgesic drug)}}{B}$

Oxalic acid on being heated up to $90^\circ C$ with conc. $H_2SO_4$ forms:

Consider the following compounds:
$(i) \, C_6H_5COCl$
$(ii) \, p-NO_2-C_6H_4-COCl$
$(iii) \, p-CH_3-C_6H_4-COCl$
$(iv) \, p-CHO-C_6H_4-COCl$
The correct decreasing order of their reactivity towards hydrolysis is:

Explain hydrogen bonding in carboxylic acids and its effect on their boiling points.