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。
The famous band had a concert __A____they played most of their original music .
A in which B which C when D that
A 因为A 和they played most of their original music 是concert的定语从句