The term "world instrument" generally refers to musical instruments that have cultural significance outside of the Western classical tradition. Based on the options you've provided, the most relevant qualification would be:
C: Instruments that are outside the Classical genre.
While A (instruments from underdeveloped countries), B (instruments from Africa and Asia), and D (instruments from Canada) may include instruments that can be classified as world instruments, they are not definitive criteria. "World instruments" can come from various cultural backgrounds and are not limited to specific geographical regions or economic classifications.