C
这是一个缩写句,完整的句子是：
This is a family which is not very big and which is one where everyone is friendly to others.
结构分析：
（1）which is not very big and which is one where everyone is friendly to others是由and连接起来的两个并列的定语从句,修饰family.两个which在各自的定语从句中都作主语,都可以换成that.但不能省略.排除D.
（2）后一个定语从句which is one where everyone is friendly to others又包含了一个定语从句where everyone is friendly to others,修饰one.这个定语从句是“主语+系动词+表语”的结构,主语是everyone,系动词是is,表语是friendly to others.可见关系词既不是主语,也不是宾语,因此不能用that,排除A、B.