What is a common test for the safety reverse function of a garage door opener?

The correct choice is placing a 2x4 under the garage door. This method is commonly used to test the safety reverse function of a garage door opener, which is designed to ensure that if the door encounters an obstruction while closing, it will reverse direction to avoid causing injury or damage.

When a 2x4 is placed under the door, the opener should detect the resistance created by the obstruction and reverse the door's direction immediately after making contact. This test effectively verifies that the safety sensors and mechanisms are functioning properly and that the door reacts as intended when an obstruction is present.

Using a piece of cardboard or a tennis ball may not provide sufficient resistance to accurately test the opener's safety features; they may not trigger the reverse function effectively. Pulling the door down manually does not assess the automatic safety features in place, as it does not simulate the scenario of an obstruction during operation. Thus, testing with a 2x4 is the recognized method for ensuring the garage door opener has a functioning safety reverse system.