Mary is a girl who _________ her mother, so if she goes abroad, she may feel homesick.
A.attaches toB.is attachedC.is attached toD.attach herself to在线课程C
解析：
很显然本题中空格所在的定语从句表达的是“她依恋她妈妈”，由于是定语从句，且以关系代词who作主语，故从句中谓语动词应与girl一致，因此D项被排除，A、B两项表达不完整。