楼主啊.你是不是把C和D打错了,有一个应该是six-year-old
答案：six-year-old
我有一个六岁的女儿.
six-year-old
做形容词,修饰daughter,连字符中间不要复数了.所以是six-year-old
如果是six years oid的话,应该是I have a daughter who is six years oid.
six years oid是不能直接作定语的.
希望帮到楼主哦.