The correct answer is B.
"In his hand he carried a wonderful hammer which always came back to his hand when he threw it."
The original sentence reads:
In his hand he carried a wonderful hammer which always had come back to his hand when he threw it.
When you read the underlined sentence, it doesn't flow together correctly. The words "had come" are easily replaced by "came." It sounds better when you read it out loud. "Had come" also changes verb tense in the middle of the sentence, which is generally a bad thing in ELA. C and D are incorrect for similar reasons. Sometimes, you can say the sentence out loud and know what fits the best.