In the heat of the movement, the government was offering rewards to anyone who would turn in key student activists.
Here, the used idiom (turn in key student activists) in the sentence means -

• In the heat of the movement, the government was offering rewards to anyone who would turn in key student activists.
- এখানে, idiom হচ্ছে turn in key student activists (turn somebody in).
- Here, the required answer is - To inform authorities about someone's wrongdoing.

• Turn somebody in
English Meaning: to take a criminal to the police.
Bangla Meaning: পুলিশের হাতে সোপর্দ করা।

Ex. Sentence: The hit-and-run driver turned himself in to the police the day after the accident.

Bangla Meaning: দুর্ঘটনা ঘটিয়ে পালিয়ে যাওয়া চালক দুর্ঘটনার পরের দিন নিজেকে পুলিশের হাতে সোপর্দ করেছে।

Source: Website Lecture.