答案：A.
翻译：这所学校学生的数量比他们学校学生数量要更大/多.
1.that为指示代词,代指前面的the number of the students,而theirs = their school.
即原句可以改写为：The number of the students in this school is larger than that in theirs.
= The number of the students in this school is larger than the number of the students in their school.
2.而B项的of mine没法理解,mine倒是可以等于my school,但是介词of有误,和前文不对应.
前句of the students,但是mine不能换成the students,只能换成my school.