But the judiciary has a rule: Even-numbered Star Trek movies have greater legal authority than odd-numbed ones.
That is an odd rule !
.
However, there are still difficult-for-computer-but-easy-on-humans tasks that can be done. I'm surprised no one's yet hooked a way into the Amazon Mechanical Turk or the like. Perhaps a simple one can be where you show a panoramic view along a busy street. Then you ask the question "What is the name of the store at number 763?" Or "What is the street number of ZZZ Supermarkets along this street?". "There is a large group of friends gathered near XXX store. How many people are in the group?"
Or simpler ones - if your forum or other thing is about a specific topic, ask a question about that topic. Or even self-referential ones. "What of the following will an art thief steal? A) Mona Lisa, B) Big screen HDTV, C) Cellphone, D) Money".
Might as well advance the state of things like image recognition and natural language queries while we're at it.
Coz with the alternatives you propose a human has to first figure out the correct answer to compare against the user's response in a CAPTcha challenge. If they had an algorithm to figure it out, the attacker would use it too. And, millions of CAPTCHAs are served everyday, so they have to be automated.
The computer is to the information industry roughly what the central power station is to the electrical industry. -- Peter Drucker