答案：D

题目分析：stand on his own feet自主，相当于depend on himself依靠某人自己，句意：当一个孩子意识到他应该独立时，他已经长大了。故选D

点评：stand up quickly快速地站起来，go somewhere on foot步行去某地，get up by himself自己起床，这些短语来说重在理解字面意思，同时解答此类题也同样，通过字面意思的来推知隐含的意思是解答此题的关键。