If Karel is instructed to only put down a ball when a specific condition is met, which control structure would be best used?

The best choice for the scenario where Karel is instructed to only put down a ball when a specific condition is met is the if statement. This control structure allows Karel to evaluate a condition and execute a specific action, like putting down a ball, only if that condition is true. If the condition does not hold, Karel simply skips the action, which is exactly what is required in this case.

Using an if statement provides an efficient way to implement decision-making in Karel's programming, allowing for clear and direct control over when to perform the action of putting down a ball. It means Karel doesn't repetitively check or execute in a loop, which wouldn't be necessary since the goal is a single action based on a condition rather than a repeated process.

Other control structures, like a while loop or a for loop, are designed for situations where actions are executed multiple times based on a condition or a set number of iterations. A do-while loop is similar but guarantees at least one execution before the condition is checked, which wouldn’t be suitable if the action should only occur based on the conditional check. In this case, where the action should only occur once based on a condition, the if statement is the optimal solution.