The European market was interested in importing cotton from India, which was a major cotton-producing country during the colonial period. However, there was also a demand for other fabrics in Europe. Let's discuss the options given in the question to understand why silk was the correct answer.

- Synthetic: Synthetic fabrics like nylon, polyester, and spandex were not available during the colonial period. These fabrics were invented in the 20th century, much after the colonial era.
- Khadi: Khadi is a hand-spun and hand-woven fabric made from cotton. It was popularized by Mahatma Gandhi as a symbol of self-reliance and non-violent resistance during the Indian independence movement. However, it was not a major export commodity during the colonial period.
- Silk: Silk is a luxurious fabric made from silk fibers produced by silkworms. India was known for its silk production since ancient times. The Europeans were fascinated by the quality and beauty of Indian silk, and it became a popular export commodity during the colonial period. Silk was used to make fancy clothes, accessories, and home furnishings.

Therefore, the correct answer is option 'C' - Silk, as it was a major market for European traders in the colonial period.