Keep in mind that it's the publisher's policy that determines the Journal Method. You don't get to choose the method.
Method A
Author publishes in a journal that submits all NIH-funded final published articles to PMC; no fee for submitting to PMC, but these journals often have required Author Publishing Fees. List of Method A journals.
Method B
Author requests the publisher submit an individual NIH-funded final published article to PMC; often fee-based service.
Method C
Author self-submits the final peer-reviewed manuscript to the NIH manuscript submission system (NIHMS); no fee.
Method D
Publisher submits the final peer-reviewed manuscript on behalf on NIH-funded authors; no fee.
Not sure what method your journal is using? Try the identification wizard or ask a librarian for more information.