强调句的用法你记住一条就行：去掉It is.that.之后,整个句子在调整完语序之后是一个完整而且通顺的句子.
如上题,去掉之后变为：the first Spring Festival in three years Xiaofeng's parents managed to come back to their hometown．
调整一下：Xiaofeng's parents managed to come back to their hometown the first Spring Festival in three years .
那么最后这个the first Spring Festival in three years 本应该作为时间状语修饰这个事件,但是少了一个介词in 或者during ,这样整个句子就不完整了,或者说多出一部分.就不对了.
关于定语从句我就不说了,看来你是懂选B为什么对的~
我说的若有不合适的地方,还请指正~