It says users already have access to view which means users already have the marketing user checked. Some of them now need access to edit. SO Permission Set (D) is correct.

I think the explanation behind option 'B' is that since by clicking the Marketing User checkbox, the user can create a Campaign, that means the user can edit the Campaign as well and hence he would have edit access through Profile only.

Permission Sets is correct
https://help.salesforce.com/s/articleView?id=sf.perm_sets_overview.htm&type=5

FYI, A Sales user is trying to manage Campaign Members for an upcoming networking event. The user can view the Campaign, but add new Campaign Members or update Member statuses. How can an administrator troubleshoot this problem?

A. Create a permission set to allow the user to edit Campaign Members.
B. Provide the user access to both Leads and Contacts to edit all Members.
C. Make sure the Marketing User Checkbox is checked on the user record page. -------correct
D. Run a Campaign report and update any Member information via Data Loader.

B isn't right because the "Marketing User checkbox" is part of the User page not the profile one

D is correct: A permission set is a collection of settings and permissions that give users access to
various tools and functions. Permission sets extend users’ functional access without
changing their profiles.

No B is right! The Marketing User checkbox allows the user to create campaigns and use the Campaign Import Wizards. If this option isn't selected, the user can only view campaigns... Kwak

Correct option should be D.