It was to be P : before their school examination Q : which was due to start R : the last expedition S : in a month’s The Proper sequence should be:

• R – the last expedition
• P – before their school examination
• Q – which was due to start
• S – in a month’s

The sentence can be rearranged as follows:

• The last expedition was to be held before their school examination, which was due to start in a month.

This is the only option that makes sense and flows smoothly. The other options are incorrect because they do not follow the correct order of the words.