Also, "pulchritude" functions to debunk the answer that they are all words that don't describe themselves, that's sort of obvious because of the fact that pulchritude is a noun and can't describe anything, technically, which means that's not the best way to word the answer. The attention to detail to place a noun on the list leaves only one best answer: "They are not themselves."
You're supposed to get hung up on s-less (shitless) by thinking, wait, it IS shitless because he threw the hyphen in there. But it remains true, because the new word means something different in this abbreviated form if read more literally; it means "lacking an 's'".
I think I got the quiz answer. What they all have in common is that they are "not themselves." I think that he meant for it to be answered idiomatically like that.