How does Karel build a tower in the Big Tower function?

Karel builds a tower in the Big Tower function by moving upward and placing balls. This method is essential because constructing a vertical tower requires Karel to ascend step by step, ensuring that each ball is placed directly on top of the previous one. The action of moving upward allows Karel to position itself correctly for each placement, creating a stable tower structure.

This approach emphasizes Karel's ability to manipulate its environment effectively by using movement in conjunction with actions to place objects. Each upward move followed by placing a ball results in an organized stack, which aligns with the definition of building a tower. The other choices do not represent the actual mechanics of tower building, as stacking horizontally would not create a vertical tower, jumping over obstacles does not pertain to ball placement, and moving backward would contradict the requirement of ascending to construct a tower.