Option 1: Micropayments, high enough to discourage bot operators but not the intended human audience will be better for website operators as soon as it becomes

Option 1: Micropayments, high enough to discourage bot operators but not the intended human audience will be better for website operators as soon as it becomes too easy for AI to solve captchas. If website operators don't explicitly introduce micropayments as a captcha alternative, there will be browser plugins that outsource captcha solving to AI for a micropayment, which has the same effect. Option 2: Using a means of authentication that can't be obtained cheaply at scale by bots, e.g. Twitter accounts, Gmail accounts, government ID, ...