A developer is working on an existing checkout implementation built against the Lightning Web Runtime...

 To implement a custom child checkout component for a B2B or B2C store created with an LWR template, the developer must implement the CheckoutStep interface from the commerce/checkout module. This interface defines the methods and properties that the child component must have to communicate with the parent checkout component and the checkout data provider. TheCheckoutStep interface extends the UseCheckoutComponent interface, which provides the common functionality for all checkout components, such as loading and saving data, reporting validity, and handling errors.

The other options are not correct because:

A.CheckoutSavable is not a valid interface for a custom child checkout component. There is no such interface in the commerce/checkout module or the Lightning web component framework.

B. Checkoutoutinterface is not a valid interface for a custom child checkout component. There is no such interface in the commerce/checkout module or the Lightning web component framework.

C. CustomCheckout is not a valid interface for a custom child checkout component. There is no such interface in the commerce/checkout module or the Lightning web component framework.

References:

Create a Custom Checkout Component for a B2B or B2C Store (LWR)

CheckoutStep Interface

UseCheckoutComponent Interface