Concept:The correct phrasal verb must show that the experiment produced a successful result after many attempts.Explanation:"Paid off" means something became worthwhile or produced good results.In this sentence, the trials led to a successful experiment, so the phrasal verb needed is "paid off".The other options do not fit: "paid up" means to pay a debt, "paid for" means to be punished or to cover the cost, and "paid out" means to spend money or release a rope.Answer:D. paid off