Answer: C or D
Technically answer C is the most formal and polite way to ask someone’s age. Answer D is more informal but also polite and is used with people you are more familiar with. If you are asking someone who is much older than you and you don’t know them well, it is best to use answer C. Also note that these two answers use the polite form of “you,” 您 (nín) instead of 你 (nǐ).
Zhāng Míng: Lǐ nǎinai, nín jīnnián duō dà suìshù le a?
张 明： 李 奶奶，您 今年 多 大 岁数 了啊？
Zhang Ming: Grandma, how old are you?
If you are speaking to a small child or someone the same age as you you can use answer A or B. This is a more informal way to ask someone their age.
Shòuhuòyuán: Xiǎo péngyou, nǐ jǐ suì le?
售货员： 小 朋友， 你几 岁了？
Salesman: Little fellow, how old are you?
Xiǎopéngyou: Wǒ bā suì le.
小朋友： 我 八 岁 了。
The kid: I’m eight years old.