答案：D

解析：

Choice (D) is correct. Let r be the number of red marbles in the jar. The probability of selecting a red marble is equal to the number of red marbles divided by the total number of marbles. Since the probability of selecting a red marble is 2/3, it follows that 2/3 = number of red marbles/total number of marbles. There are 36 green marbles and r red marbles, so the previous equation becomes 2/3 = r/(r + 36). This simplifies to 2(r-36) equals 3r, or (2r)-72 equals 3r. Therefore, the number of red marbles, r, is 72.