How would you decide this?

Dave Perry’s 100 Best Racing Rules Quizzes highlights specific aspects of the racing rules in a fun format designed to help you become more familiar with The Racing Rules of Sailing. Here is one of the quizzes:

Quiz 37
Boats P and S, two close-hauled boats on opposite tacks, are approaching a windward mark to be left to starboard. S hails, “Starboard!” P tacks to keep clear and protests S for not tacking and sailing her proper course to the mark.

You are on the protest committee…how would you decide this? Answer below.

Quiz 37 Answer:
Disallow P’s protest. Because the boats are on opposite tacks, P is required to keep clear of S under rule 10, On Opposite Tacks. Though the boats are in the zone of the windward mark, rule 18, Mark-Room, does not apply because the boats are on opposite tacks on a beat to windward (see rule 18.1(a)(1), When Rule 18 Applies) and because the proper course for S but not P is to tack (see rule 18.1(a)(2)). No rule requires S to sail her proper course in this situation. (See World Sailing Case 9.)

Source: US Sailing