In a certain code language, ‘join the class’ is written as ‘yq pt dm’ and ‘the music class’ is written as ‘rj yq dm’. How is ‘music’ written in the given language?

Let's compare the given coded sentences to extract the code for each word:
1. 'join $\underline{\text{the}}$ $\overline{\text{class}}$ ' = 'pt $\underline{\text{yq}}$ $\overline{\text{dm}}$ '
2. '$\underline{\text{the}}$ music $\overline{\text{class}}$ ' = '$\underline{\text{yq}}$ rj $\overline{\text{dm}}$ '

* The words 'the' and 'class' are common to both sentences. Looking at the codes, 'yq' and 'dm' are also common. Therefore, the common words '{the, class}' map to the codes '{yq, dm}'.
* The only remaining word in the second sentence is 'music', and its corresponding remaining code is 'rj'.

Thus, ‘music’ is written as ‘rj’.