Karpas is whatever vegetable a household or region has readily available in early spring, parsley and celery are the most familiar choices today, but potato became the standard in much of Eastern Europe, where fresh green vegetables were simply not reliably available at the time of year Passover falls, a clear case of ritual practice adapting to what the climate could actually supply.
Preparation
Whichever vegetable is used, preparation is minimal, a small piece cut and dipped in salt water prepared fresh for the meal, work any home cook can do with no special training or restriction on who performs it.
Its job at the table
Karpas is eaten early in the seder specifically because dipping a vegetable in salt water this soon in a meal is unusual enough to prompt a question from anyone at the table encountering the ritual for the first time, particularly children, which is exactly the purpose it is built to serve within the broader structure of a seder designed around invited questions. Like the other fresh ritual foods on this list, it is simply eaten and leaves no disposal question behind it.
A choice some households still debate
Because no single vegetable is mandated, some households alternate between celery and parsley from year to year, or use whichever looks freshest at the market that week, treating the specific choice as a matter of custom and convenience rather than as a fixed requirement the way the minimum quantity of maror is fixed. The salt water itself carries its own regional variation too, some families use vinegar in its place where salt was historically scarce or expensive, another small adaptation of the same underlying ritual to whatever the kitchen actually had on hand.