To solve the problem where Karel needs to take all the tennis balls from the spot where he is standing, a While Loop would be the most appropriate control structure.
This is because Karel should continue picking up tennis balls as long as there are tennis balls present. A while loop allows Karel to repeatedly execute the action of picking up a tennis ball until there are none left at that location.
Here's a brief example of what this might look like in pseudocode:
While there is a tennis ball on the current spot:
take a tennis ball
So, the correct answer would be While Loop.