Answer: C. Semiconductor materials.
Answer analysis: This question mainly tests the understanding of the application of semiconductor materials.
Option A: Ceramic materials are usually used as insulators, which play the role of isolating current in circuits. They do not have the characteristics of light-emitting diodes, so option A is wrong.
Option B: Metal materials are good conductors of electricity. They allow current to pass freely, but they do not have unidirectional conductivity and luminescence characteristics. Therefore, they are not the main materials of light-emitting diodes. Option B is wrong.
Option C: The conductivity of semiconductor materials is between conductors and insulators. They have unique electrical properties, such as unidirectional conductivity, photosensitivity, and thermal sensitivity. Light-emitting diodes are made of semiconductor materials. They use the characteristics of semiconductor materials to emit light under forward voltage and not under reverse voltage. Therefore, option C is correct.
Option D: Nanomaterials refer to materials that have at least one dimension in the nanoscale range (1-100nm) in three-dimensional space or are composed of them as basic units. Although nanomaterials have many unique physical and chemical properties, they are not the main materials of light-emitting diodes, so option D is wrong.
In summary, the correct answer is C.