Final answer:
The term used by an umpire when a player enters a restricted area on the court is encroachment. This is a type of penalty that leads to consequences such as yardage gains for the opposing team, unlike trespassing, obstruction, or a warning which have different meanings.
Step-by-step explanation:
When a player enters an area on the court that they're not meant to be in, the correct term that the umpire would use is c) Encroachment. This term is commonly applied in various sports to describe a situation where a player illegally enters a specific zone or crosses a boundary that they are not allowed to be in during play. An example of encroachment in football would be a defensive player crossing the line of scrimmage before the ball is snapped. In this case, it is considered a penalty, and the opposing team is awarded a certain number of yards as a consequence. The other options, such as trespassing, are not sports-related terms in this context; obstruction refers to impeding the movement of an opponent, and a warning is generally a caution issued by the umpire for various infractions but is not specific to illegal positioning.