A,in which 中which就指代a concert,即in the concert they played most of their original music.
翻译：这个有名的乐队在演唱会上（in the concert）演唱了他们大部分的原创歌曲.
in which=where 是定语从句的引导词,后面部分做a concert的定语.为什么不能选that 呢???that不是可以也指代的吗?因为用that的情况是后面的定语从句缺少主谓宾的成分，你看后面主谓宾都不少，所有肯定是少状语之类的，in which 就表示in the concert，that不能指代in the concert，只能指代concert