The issue described involves problems with submitting coupon codes and users being logged out upon attempting this action, which strongly points to issues related to Cross-Site Request Forgery (CSRF) protection mechanisms in Salesforce B2C Commerce Cloud.

Given the described behavior:

Option A suggests verifying if the CSRF cartridge is in the cartridge path, which is essential but not directly related to the specific issue of form submission.

Option B discusses the 'secure' attribute of the form group, which is important for HTTPS security but does not address the CSRF token issue directly.

Option C is the correct answer, as it focuses on verifying if the CSRF token is present in the form and is being submitted with the request. This token is crucial for validating that the form submission is legitimate and not a CSRF attack. The absence or mishandling of this token could lead to the issues described, such as users being logged out when they try to submit a form, as the system might interpret these actions as potential CSRF attacks.

Option D suggests checking the CSRF settings in Business Manager, which, while important, is more about configuration rather than troubleshooting specific form submission issues in the code.