Concept:A preposition must correctly connect the verb to the phrase showing purpose.
Explanation:The phrase “were joined in make” is incorrect.
The preposition “in” cannot introduce the purpose of joining the benches.
The correct phrase is “were joined to make”.
Here, “to make” correctly shows the purpose of the action.
So the error is in the segment “were joined in”.
The corrected sentence is: “The play was staged in the central courtyard of the school and benches were joined to make a temporary stage.”
Answer:Option C: “were joined in”