Cloud Kicks (CK) wants to set up a custom child object to track gift cards...

The app builder should create a master-detail relationship field on the gift card object to support the requirement. A master-detail relationship field creates a parent-child relationship between two objects, where the master record controls certain behaviors of the detail record. In this case, the Account object would be the master and the gift card object would be the detail. By creating a master-detail relationship field, the app builder can meet these requirements:

Track the total number of gift cards opened and gift cards issued on an Account: This can be done by creating roll-up summary fields on the Account object that count or sum up the values from the gift card records related to each account.

Permanently ensure the gift cards are unable to be moved across any other Account once it is created: This can be done by making the master-detail relationship field required on the gift card object, which means that every gift card record must be associated with an account record and cannot be changed once set. Option B, C, and D are not types of fields that can support this requirement.