The correct answer is option a. "he thinks a magician has turned giants into windmills".
Explanation:
Don Quixote's multiple illusions that he saw in his travels where constantly attributable to a magician changing the reality that he was seeing. This was the case when Don Quixote believed that he was facing giants in the skirmish. After Sancho Panza watched how his master was facing windmills, Don Quixote attribute his loss of the skirmish to a magician that had turned giants into windmills.
