本题的难点是它涉及比较专业的音乐知识。仅从语法角度来看，它其实非常简单。

The distance by which these accidentals change the pitch is called a semitone.

by which 引导定语从句，即by which these accidentals change the pitch 是修饰 distance 的定语从句。which 是关系代词，指代先行词 distance。

句子大意：这些升降符号改变音高的距离称为半音。（只是大意，不一定符合音乐的专业表达）

下面是几个含有 by which 引导定语从句的例子，你可以体会和比较一下：

Ratings are the yardstick by which TV shows are evaluated by networks. 收视率是产播电视网衡量电视节目的标准。

We don’t have a common yardstick by which to compare the two cases. 我们没有一个共同的标准来比较这两种情况。

Testing is still the usual means by which students’ progress is measured. 考试仍是衡量学生进步程度的惯用手段。

The couple have an arrangement by which they each contribute equally to the cost of the house. 夫妇双方协议，平均分担购房的费用。

She says we need to depoliticize the process by which judges are chosen. 她说我们应该让选备法官的程序不受政治影响。