In SFCC, Salesforce Commerce Cloud, there is a dedicated section for services. In which you can create services with some special features.
You can find services under Administration > Operations > Services

There are three components in which Services is divided in SFCC, Services, Profiles & Credentials.

In Credentials Section, here you will find 2 fields, User and Password where username and password for the service is saved.

For this Question there are 4 options:
a. In JSON Config File
b. Custom Preferences on System Preferences
c. Service Credential for Current Service
d. Create custom attribute for web service to save it.
Now we know that it is created in Credentials section under service, hence the correct answer is option c.
Stay tuned for more such questions. Lets learn and grow together.