Step 1: Recall the recipe for making an ester.
A pleasant smelling ester is formed whenever a carboxylic acid is heated together with an alcohol in the presence of a little concentrated sulphuric acid acting as catalyst. \[ \text{R-COOH} + \text{R'-OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{R-COOR'} + \text{H}_2\text{O} \]
Step 2: Identify what is already given.
The alcohol part is already fixed for us, ethanol, and sulphuric acid is already supplied as the catalyst, so we only need to pick the carboxylic acid from the options.
Step 3: Scan the options for a carboxylic acid.
Ethanol and methanol are themselves alcohols, and ethanal is an aldehyde, none of which are carboxylic acids. Acetic acid, $\text{CH}_3\text{COOH}$, is the only carboxylic acid listed.
Step 4: Confirm the product.
Heating acetic acid with ethanol and sulphuric acid produces the fruity smelling ester ethyl acetate: \[ \text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O} \] \[ \boxed{\text{Acetic acid}} \]