A total eclipse of the sun occurs when the moon is between the sun and the earth. During this event, the moon blocks the sun's light completely or partially, causing a shadow on the earth's surface. Therefore, option C, "moon is between the sun and the earth," is the correct answer. Option A, "earth is between the moon and the sun," describes a lunar eclipse, and option B, "sun is between the moon and the earth," is not possible as the moon cannot pass behind the sun. Option D, "ozone layer is threatened," is unrelated to the occurrence of a total eclipse of the sun.