What action must an umpire take if a bat is found to be modified?

When an umpire identifies that a bat has been modified, the appropriate action is to confiscate the bat and issue a penalty. This is crucial because using a modified bat poses a significant risk to player safety and undermines the integrity of the game. Modified bats may provide unfair advantages, potentially leading to excessively dangerous situations due to increased performance beyond allowable equipment standards.

By confiscating the bat, the umpire ensures compliance with the rules governing legal equipment in the context of competition. Issuing a penalty serves as a deterrent to both the offending player and the team, reinforcing the seriousness of adhering to equipment regulations. It helps maintain a level playing field, ensuring that all players compete under the same conditions and with equipment that meets the standards mandated by the governing body of the sport.

The other options do not adequately address the violation of the rules or ensure the safety and fairness that the regulation aims to protect. Allowing the bat's use, replacing it with another, or merely informing the coach fails to confront the serious implications involved with modified equipment.