The word 'smolder' means burn slowly without a flame.
Smolder most nearly means burn slowly and without a flame. This is evident in descriptions like 'a smouldering unclean yellow,' where the slow-burning nature is highlighted.
For example, in the sentence 'The color is repellent, almost revolting; a smouldering, unclean yellow, strangely faded by the slow-turning sunlight,' the use of 'smouldering' refers to a slow burn without a flame.
Therefore, the correct choice is C. burn slowly and without a flame.