How many different carboxylic acids are obtained when all the isomeric arenes having the molecular formula ${\mathrm{C}}_8{H}_{10}$ are oxidized with alk. ${\mathrm{KMnO}}_4$  followed by acidification?

Isomeric carboxylic acids are formed from four different arenes:

ethylbenzene $\to$ benzoic acid, 

o-xylene $\to$ phthalic acid, 

m-xylene $\to$ isophthalic acid 

and p-xylene $\to$ terephthalic acid.