What structure allows Karel to repeat an action until a condition evaluates to false?

The while loop is designed specifically for scenarios where an action needs to be repeated as long as a certain condition remains true. This makes it ideal for situations where you don't know in advance how many times an action should be performed.

When Karel encounters a while loop, it evaluates the condition each time before executing the block of code within the loop. If the condition evaluates to true, Karel will execute the code inside the loop and then reevaluate the condition. This process continues until the condition evaluates to false, at which point Karel exits the loop and moves on to the next part of the program.

This structure is particularly useful when you want Karel to keep performing actions, such as moving forward or picking up beepers, until a certain situation changes, like reaching a wall or running out of beepers. Other options, such as the if statement, are used for making decisions based on conditions but do not provide repetition. The for loop has a predetermined number of iterations, and the return statement is used to exit a function or indicate the end of a procedure, not for repeating actions.

Hence, the while loop is the most appropriate structure for performing repetitive actions based on a condition in Karel's programming.