The lengths of a triangle must satisfy the triangle inequality which states that the sum of any two sides must be greater than the third side.
For option A:
5 + 9 = 14, which satisfies the triangle inequality. However, 5 + 14 = 19, which is not greater than 9. Therefore, this set of numbers does not satisfy the triangle inequality.
For option B:
7 + 7 = 14, which is less than 15. Therefore, this set of numbers does not satisfy the triangle inequality.
For option C:
1 + 2 = 3, which satisfies the triangle inequality. However, 1 + 4 = 5, which is not greater than 2. Therefore, this set of numbers does not satisfy the triangle inequality.
For option D:
3 + 6 = 9, which satisfies the triangle inequality. 3 + 8 = 11 and 6 + 8 = 14, both of which are greater than 8. Therefore, this set of numbers satisfies the triangle inequality.
So, the numbers 3, 6, and 8 could represent the lengths of a triangle. Therefore, the answer is option D.