When propionic acid is reacted with aqueous sodium carbonate,$CO_2$ is released. From where does the $'C'$ of $CO_2$ come?

Which of the following can undergo Hell-Volhard-Zelinsky reaction?

The following species are involved in the formation of an ester from a carboxylic acid in the presence of acid. The correct sequence of formation of these species is

The refluxing of $(CH_3)_2NCOCH_3$ with acid gives

Consider the acidity of the following carboxylic acids. Which of the following orders is correct?
$A$. $PhCOOH$
$B$. $o-NO_2C_6H_4COOH$
$C$. $p-NO_2C_6H_4COOH$
$D$. $m-NO_2C_6H_4COOH$

The product $(D)$ in the following reaction sequence is: $HOCH_2CH_2CH_2COOCH_2CH_3$ $\xrightarrow{PCC} (A)$ $\xrightarrow[{(1 \ molar \ equivalent)}]{H_2C=CHMgBr} (B)$ $\xrightarrow{NH_4Cl/H_2O} (C)$ $\xrightarrow[H_2O]{KOH}$ $\xrightarrow{H_3O^{+}}$ $\xrightarrow[pyridine]{(CH_3CO)_2O} (D)$