In this scenario, you want Karel to move only if the front is clear. If the front is not clear, Karel will do nothing. The appropriate control structure for this is:
If Statement
An if statement allows you to execute a block of code (in this case, Karel's movement) only when a certain condition is true (in this case, when the front is clear). If the condition isn't met (when the front is not clear), Karel does nothing, which aligns perfectly with your requirements.
So, the most suitable choice is:
If Statement