Option C is the answer.
Option A is incorrect as the apostrophe indicates possession.
Option B is incorrect as the same reason as above.
Since R is capitalized, a single 's' will be added without any apostrophe. If R weren't capitalized, then it should have been 3 r's so as to avoid confusion of whether the writer is talking about rs or only r.
R's means "of R".
Hope I helped! You may comment here or message me if you have any further doubts :)
