Here's why:

* Meta-xylene has its two methyl groups in the 1,3 positions on the benzene ring. When a third substituent is added, it can only go to the remaining position (either 2, 4, or 5) resulting in a single trisubstituted product.

Let's look at the other isomers:

* Ortho-xylene has its two methyl groups adjacent (1,2 positions). Adding a third substituent can lead to two different trisubstituted products depending on whether it's placed on the carbon adjacent to both methyl groups or the carbon opposite one of the methyl groups.

* Para-xylene has its two methyl groups opposite each other (1,4 positions). Adding a third substituent can result in two different trisubstituted products depending on whether it's placed adjacent to one of the methyl groups or opposite to both.

Therefore, meta-xylene is the only isomer that results in a single trisubstituted benzene product.