Three actions that a developer must take, in a B2B Commerce store, to accept credit card payments using a client's chosen payment provider are: create a named credential for authentication with the payment provider, create an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface, and create a PaymentProviderGateway record for the custom payment provider class. Creating a named credential for authentication with the payment provider allows the developer to securely store and manage authentication information, such as username, password, token, or certificate, for connecting to the payment provider's API or service. Creating an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface allows the developer to define custom logic for processing credit card payments using the payment provider's API or service. The interface provides methods for validating credit card information, authorizing payments, capturing payments, voiding payments, and refunding payments. Creating a PaymentProviderGateway record for the custom payment provider class allows the developer to register the custom payment provider class as a payment gateway in the store and associate it with a payment method. Creating a RegisteredExternalService record for the custom payment provider class is not a required action, as this is only used for registering external services that are not related to payment processing, such as tax or shipping services. Creating an Apex class that implements the commercepayments.PaymentGatewayAdapter interface is not a required action either, as this is only used for D2C Commerce stores, not B2B Commerce stores. Salesforce References: B2B Commerce Developer Guide: Payment Integration, B2B Commerce Developer Guide: Payment Gateway Adapter Interface, B2B Commerce Developer Guide: Payment Provider Gateway Object

Three actions that are applicable when modifying the number of steps required in the Salesforce Commerce Checkout flow are:
Perform a template override on the Checkout page. This action will allow you to change the structure and content of the Checkout page, such as adding or removing sections, widgets, or fields. For example, you can override the checkout.handlebars template and modify it according to your requirements.
Set the value of the configuration setting defined as CO.overrideFlow to TRUE. This setting will enable you to use your own custom checkout flow instead of the default one. You need to set this value to true before you can modify the checkout flow.
Set the value of the configuration setting defined as CO.useDef to TRUE. This setting will enable you to use a single-page checkout flow instead of a multi-step checkout flow. You need to set this value to true if you want to reduce the number of steps in the checkout flow to one. Salesforce Reference: B2B Commerce and D2C Commerce Developer Guide, Checkout Flow

Explanation
A valid way of referencing the global cc_api_CartExtension apex class via subscriber code is to use ccrz.cc_api_CartExtension. This is the name of the class that is defined in the cloudcraze managed package.
The class is global, so it can be accessed by subscriber code. The other options are either invalid or incorrect.
Salesforce References: B2B Commerce and D2C Commerce Developer Guide, API Classes

Explanation
The event that is triggered within Salesforce B2B Commerce whenever a cart's state changes is cartChange.
This event is triggered by the updateCart method of the CCRZ.Cart class, which is responsible for updating the cart data and performing any actions that depend on the cart state, such as applying discounts, taxes, or shipping charges. The event can be used to notify all the components that are listening to it that the cart has changed and they should refresh their data accordingly. Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Events, Cart Class

To indicate that inventory is available for all items and amounts in the cart, the developer should return sfdc.checkout.InventoryStatus.Status.SUCCESS. The sfdc.checkout.InventoryStatus.Status.SUCCESS is a constant value that represents a successful inventory status. It means that the inventory class has checked the availability of all the products in the cart and confirmed that they are in stock and can be fulfilled. The developer should use this value as the return value of the checkInventory method, which is a method that implements the sfdc.checkout.InventoryCheck interface. The checkInventory method is invoked during the checkout flow to validate the inventory availability of the products in the cart. Returning TRUE is not a valid way to indicate inventory availability, as it is not a valid inventory status value. Returning sfdc_checkout.IntegrationStatus.Status.SUCCESS is not a valid way either, as it is not an inventory status value, but rather an integration status value. Returning sfdc.checkout.InventoryStatus.SUCCESS is not a valid way either, as it is not a valid syntax for accessing the inventory status value. Salesforce Reference: B2B Commerce Developer Guide: Inventory Integration, B2B Commerce Developer Guide: Inventory Status Enum, B2B Commerce Developer Guide: Inventory Check Interface