The correct answer is (Choice D) harvested.
The sentence should read: "Although European pears sometimes change color after being harvested, but Asian pears do not." The word "but" is unnecessary in this construction because "although" already introduces a contrast. The correct version would omit "but," making the sentence flow better.
So, the revised sentence would be: "Although European pears sometimes change color after being harvested, Asian pears do not."