The functional group R–COO–R' represents an ester. Esters are prepared by the reaction between a carboxylic acid (RCOOH) and an alcohol (R'OH). This reaction is known as esterification: \[ \text{RCOOH} + \text{R'OH} \xrightarrow{H^+} \text{RCOOR'} + H_2O \] So, the correct reactants for forming an ester are RCOOH and R'OH.
The correct answer is option (A): RCOOH and R'OH