A structural isomer is a compound that has the same number of atoms but is arranged differently. Therefore, to know which will be the structural isomer of the compound they give us, we must count the atoms of each compound.
When comparing the compounds we see that pent-2-yne has C5H8 just like 3-methylcyclobutene, therefore these two compounds are structural isomers.
The answer will be: 3-methylcyclobutene