Final answer:
The word 'so' correctly completes the sentence by expressing the cause-and-effect relationship between the miller's daughter giving the manikin her necklace and him spinning straw into gold.
Step-by-step explanation:
The correct answer to the sentence completion question is D. so. The complete sentence should read as follows: The miller's daughter gave the manikin her necklace, so he spun the straw into gold. This sentence is suggesting a cause-and-effect relationship where the daughter's action of giving her necklace leads to the manikin's action of spinning straw into gold. The word 'so' is used to express the result of the first action in the sentence.