Which of the following is a rule of tennis?

In tennis, one of the fundamental rules is that a player must win a game by a margin of at least two points. This means that if the score reaches deuce, which is a tie at 40-40, a player must win two consecutive points to win the game. This rule is in place to ensure that the victory is clear and definitive, preventing a player from winning by a mere one-point advantage.

The other options do not accurately reflect the established rules of tennis. For instance, serving may be done either underhand or overhand, so there isn't a requirement to serve in a specific manner. Games in tennis aren't played to 15 points; instead, the points are counted as 15, 30, 40, and then a game win, but the concept of points doesn't equate to a total of 15 to win a game. Lastly, a match in tennis typically consists of a predetermined number of sets, commonly best of three or five sets, not just one set.