Who do I email about a question on a paper? I want to ask the authors of a paper if they have thoughts on an application of their work. I'm not sure who to email.

The first author is a grad student, who is supervised by the last author - however, the authors are in alphabetical order so I'm not sure if there is any meaning to the ordering. The paper is in economics, which traditionally goes alphabetically, I believe.

Should I email all the authors in one bulk message? Just the professors? Choose one of the authors at random and email them?
Papers usually have one (or more) designated corresponding author, or “contact author”, or “author to whom correspondance should be addressed”. That's whom you should contact.

Sometimes it's indicated by a footnote, an asterisk, or at the end of the paper. Sometimes, it's indicated by the fact that only corresponding authors have their email address listed.

If there is no corresponding author, then email either the first author or senior author, depending on the nature of your question and how confident you feel in communicating with them. I would refrain from copying the email to all authors (except maybe if there are only two): if he wants to, the author you will write to can forward your query to others.