The blank in the sentence likely refers to a comparison of well-being or adjustment between two groups of students based on the topic of their essays. The logic seems to suggest that writing about personal challenges and processing emotions related to adjusting to college might have a therapeutic or beneficial effect on students' mental health or adjustment.
Given this, the most fitting option among the choices provided seems to be:
a. healthier
This answer aligns with the idea that reflecting on and processing personal challenges through writing could potentially promote better adjustment and mental health among college students. However, it's important to note that the real impact might vary among different individuals and under different circumstances.