The guide said to the swimmer, "Don't swim out too far."

The correct options is (D) because other options deviate from grammatical rules and are structurally faulty. The word "forbid" is negative and its meaning is "to order someone not to do something" or "to say that something is not allowed." Some example sentences with "borbid". her father forbade the marriage; you are forbidden to leave; she knew her mother would forbid her going lack of spae frobade them from mentioning the subject again. So the use of "to" after "forbade" is grammatically and syntactically allowed in the answer option (D). Furthermore there's no use of "not" in the answer option.