Which command would be used to check if Karel can move forward?

To determine if Karel can move forward, the option "if (frontIsClear())" is the most appropriate command. This command checks whether there is an obstacle directly in front of Karel. If the front is clear, Karel can move forward safely; if there is an obstacle, the command will evaluate to false, ensuring that Karel doesn't attempt to move into a wall or other blockage.

The reason this command is specifically suited for checking movement is that it directly assesses the state of the space in front of Karel, allowing for appropriate decisions based on that condition.

The other options may refer to various checks or concepts, but they do not specifically verify whether Karel's forward path is unobstructed in a manner that directly correlates to movement. Therefore, using "if (frontIsClear())" is the correct and most effective approach to verify if Karel can proceed forward.