It's exciting is to thrill as difficult is to?

This is an analogy that you need to complete in order to see what the final word should be. So, exciting is an adjective that describes the noun thrill. So, in order to finish the analogy, you need a noun that the adjective difficult refers to. That noun could be ordeal, hardship, trouble, torment, torture, etc. You need a word that means "something difficult," so any of those words could be the correct answer.