What immediate action should an operator take to stabilize an excavator as it tips?

Lowering the boom is the correct action to take when an excavator begins to tip. This maneuver helps to lower the center of gravity of the machine and increases stability. When the boom is lowered, the weight distribution shifts downward, which can counteract the tipping motion and help to stabilize the excavator.

The other options may seem logical at a glance but do not effectively address the immediate need for stability. Speeding up the engine could potentially reduce control or exacerbate the situation by creating more momentum that could lead to further tipping. Releasing hydraulic pressure would not provide immediate stability and might instead cause the attachment to drop or become uncontrollable. Pulling up on the control sticks could lead to unintended movements of the machine, which may worsen the tipping situation rather than stabilize it. Thus, lowering the boom is the most effective immediate action to take in this scenario.