In UML use case diagrams, when specifying the number of actors involved in a use case, a multiplicity marker can be used. The multiplicity marker is a number placed near the actor symbol that denotes how many instances of the actor are involved in the use case.
Option C correctly uses a multiplicity marker of '2' next to the actor 'Player', which indicates that exactly two instances of 'Player' are involved in the "Play Tennis" use case.
Here is a brief explanation of why the other options are incorrect:
A) Option A shows two separate 'Player' actors involved in "Play Tennis" without a multiplicity marker, implying possibly different kinds of players, but does not specify that exactly two instances are required. B) Option B shows two 'Player' actors involved in "Play Tennis" without a multiplicity marker, also not specifying the exact number of instances required. D) Option D uses a multiplicity marker, but it is incorrectly placed near the 'Play Tennis' use case instead of the 'Player' actor, which does not follow UML notation for indicating actor multiplicity.
Therefore, the correct answer is:
C: Option C