The correct option is C Enclosure
In the first paragraph, the sentence "At the center was the enclosure known as Altis, dedicated to Zeus, the god of the gods", it is mentioned that Altis was the name of the enclosure, making Option D the correct one. Option A is incorrect since the name of the god mentioned was Zeus and not Altis, as given in the passage, "dedicated to Zeus, the god of the gods". Option B isn't correct since the name of the race mentioned was the Greek race as mentioned in the line "They were a great national festival of the entire Greek race". In similar fashion, Option C too isn't the answer, since the festival is the Olympic Games as mentioned here: "The fame of Olympia rests largely upon Olympic Games. They were a great national festival...".