364. Here’s the explanation.

This is becomes a self-referential paradox. Both A and D would be correct if there were four unique answers, but since A and D are the same answer, the chance that you would choose a correct answer is 50%, which makes B correct. But if there’s only one correct answer, the odds of choosing the correct one at random goes back to 25%. And around and round you go.

There’s a lot of discussion at Richard Wiseman’s blog and more at Lifehacker, where I first saw this.

Sequoia, eulogia or miaoued are all good options, but there’s an even shorter (and more obscure) word with all the vowels and not a single consonant: Iouea. It’s not the kind of word you’d use at the dinner table, but it’s listed in the Wikipedia dictionary as a genus of Cretaceous fossil sponges.

If you got really tricky and tried to find a word that contained the letters in the phrase ‘all of the vowels’, you probably found, like I did, that there isn’t one. But it was still worth checking.