It's a logical brain teaser. First off, it's supposed to be "stain" not "Spain". And the thing that they all have in common is that they all can be changed into names of countries by changing only one letter. Like so... Stain (as we've seen already) becomes Spain, Tuba becomes Cuba, Chime becomes Chile, Iron becomes Iran, and Perk becomes Peru.
bars
fdsdgdg
They make a trick question.
Fire
they all have vowels
Organize
e
They all start with "D".
water
flowers
They all rhyme
Dime and Chime