Which of the following, on side-chain monochlorination with $\mathrm{Cl}{}_2$/light (no ring substitution), give only one constitutional isomer of the monochloro product?

Toluene has one set of side-chain H, giving benzyl chloride. In p-xylene the two methyls are equivalent, giving 4-methylbenzyl chloride. In tert-butylbenzene all nine H are equivalent, giving one product. Ethylbenzene has benzylic and terminal H, so two isomers form.