Which command checks Karel's current position on the y-axis?

The command that checks Karel's current position on the y-axis is represented as currentPosition.y. This notation directly accesses the 'y' property of the 'currentPosition' object, which is a common approach in programming to retrieve specific attributes of an object.

When working with objects, accessing properties is typically done using the dot notation, as seen in this correct option. This syntax allows Karel to perform the check efficiently, providing the exact value of the y-coordinate in its current position.

The other options do not conform to the appropriate way of accessing the y-axis position for Karel, either by not using the correct syntax for property retrieval or by not being standard methods typically found in programming languages used for these types of commands. This reinforces the understanding that attributes of objects are accessed through properties rather than as standalone commands or functions.