An organic compound 'A' on oxidation with pyridinium chlorochromate...

The aldehyde with the molecular formula $\text{C}_2\text{H}_4\text{O}$ is ethanal ($\text{CH}_3\text{CHO}$).

Pyridinium chlorochromate (PCC) is a mild oxidizing agent that selectively oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids.

Since the product is a two-carbon aldehyde (ethanal), the reactant 'A' must be a two-carbon primary alcohol.

Therefore, compound 'A' is ethanol ($\text{CH}_3\text{CH}_2\text{OH}$).

The reaction is: $\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{PCC}} \text{CH}_3\text{CHO}$

Answer: Ethanol