many years later, they began to miss the fesitivals,____they could get together with their families

很多年后，他们开始怀念那些的节日，那时能和家人聚在一起。
后面的句子不缺成分，所以which 不行，that不能用于非限制性从句，as意义上不行。故选A

as作为连接词 conj.有以下意思

1.像...一样;依照;像
You ought to do as Paul tells you.
你应按照保罗吩咐的做。
The work is not so difficult as you imagine.
这工作不像你想像的那么困难。
2.当...时
As she left the room she remembered that book.
她离开房间时想起了那本书。
3.随着
As the sun rose the fog dispersed.
太阳一出来,雾随之消失。
4.因为
We didn't know what to do as we were just visiting there.
我们不知道该怎么办,因为当时我们仅仅在那里作访问。
5.虽然
Tired as he was, he sat up late.
他虽然疲倦,可仍然很晚才睡。

选A，用when，作状语，引导定语从句
只有引导词在从句中做主语或宾语时，才选 B.Which 。很明显本句不是这种情况。本句的引导词在句中做时间状语。

A