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

The correct answer is B. Master-detail relationship.

Cloud Kicks needs two things: count Gift Card records on the Account and prevent Gift Cards from being moved to another Account after creation. A master-detail relationship supports both requirements.

First, master-detail enables a roll-up summary field on Account to count related Gift Card records. Salesforce documentation confirms that roll-up summaries work from detail records up to the master record.

Second, master-detail relationships prevent reparenting by default. Salesforce documentation states that records cannot be reparented in master-detail relationships by default, although admins can allow reparenting if they choose.

A is incorrect because a lookup relationship does not support standard roll-up summary fields and does not permanently lock the parent Account by default.

C is incorrect because roll-up summary is a field on the parent Account, not the relationship field type on Gift Card.

D is incorrect because a formula cannot count related child records or enforce parent locking.