What is the opposite of the action 'do'?

The correct choice, which is 'undo,' represents the action that is opposite to 'do' because it implies reversing or negating an action that has already been established or completed. When you 'do' something, you are performing an action or bringing about a result. To 'undo' that action is to take steps to negate or eliminate that result, effectively reverting to a previous state. This creates a clear conceptual contrast between initiating an action and retracting that same action, making 'undo' the most fitting opposite of 'do'.

The other options do not serve as direct opposites to the action of 'do.' For instance, 'complete' implies finishing an action rather than reversing it, while 'start' signifies the beginning of an action and 'finish' denotes bringing an action to an end. None of these terms convey the idea of negation that 'undo' does.