Which of the following can cause a penalty on the putting green?

On the putting green, the only situation from the choices presented that can cause a penalty is related to loose impediments. According to the Rules of Golf, players are allowed to remove loose impediments on the putting green without penalty. However, mishandling or improperly removing loose impediments can lead to a penalty if the actions disrupt the playing area or interfere with another player’s ball.

Considering the other choices, markers indicating the hole typically do not result in a penalty since they are part of the course setup and not directly affecting play in a manner that would incur a penalty. Ridges or bumps on the green are expected features of the surface and do not lead to a penalty; they are simply part of the play conditions. Unattended flags are also not a cause for penalties, as a flagstick left unattended does not violate any specific rule regarding play on the green. Thus, loose impediments stand out as the correct answer due to their specific handling and potential for infraction under the rules.