The correct answer is C. very cheaply.
The phrase “he sold his house for a song” means that he sold it very cheaply. This is because a song is a very short piece of music, and therefore not worth very much. The other options are incorrect because they do not mean that he sold his house very cheaply. Option A, “at a reasonable price,” means that he sold it for a price that was fair and not too expensive or too cheap. Option B, “at a discount,” means that he sold it for less than the original price. Option D, “at a premium,” means that he sold it for more than the original price.