Starting as a ________campus movement, initially observed on March 21, Earth Day has become a major educational and media event.
句意：地球日最初是由学生领导的校园运动，最初于3月21日开始，现在已经成为一项重要的教育和媒体活动。student-led是led by students的形容词形式，意为“由学生领导的”。因此，本题的正确答案为C。