C 、the one
题干缺表语,所以用the one(= the city)
定语从句 your elder sister was born in 缺宾语,故排除不能作宾语的关系副词B.where
如果选A,或者D ,那么（that/ which) your elder sister was born in这个定语从句便是修饰主语 this city 了.这样的话,还原为陈述句语序立马可见句子成分残缺（缺表语）：
This city （that/ which) your elder sister was born in is.
如果选the one 便是：
This city is the one（that/ which) your elder sister was born in.
所以是对的!