Anyone with a half functioning brain can figure out which ones are the taxis. Taxis are not yellow in my country and I would have no issue picking those out. They have the very distinctive sign on the roof that no normal car has. Not even OP was confused by this, they are just complaining on the behalf of some theoretical person who might get confused.
Testing some more here https://www.google.com/recaptcha/api2/demo I also got "mark all fire hydrants". I've never seen one in real life, only those red in American cartoons. I got "parking meters", but none of them looked like they do in my country (and I've never heard anyone call them that word they used for translation). I got mark "mountains", but to quote Dundee: You call that a mountain?