The question is "Which of the following, if true, most strongly supports the hypothesis that the book was produced in 1148". If "produced in 1148" means "completed in 1148" then none of the choices substantiate that.We can only say from D that the fourth copyist existed in 1148 and may have worked on it in that year. How can we conclude he completed it? D looks like the best answer, but I thought that the GMAT answers are supposed to be "CORRECT" not just BEST.