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