Marketing Cloud should be the single source of truth for customer journey flow, which is the sequence of interactions that a customer has with a brand across different channels and touchpoints. Marketing Cloud can help design, execute, and optimize customer journeys using Journey Builder, which can leverage data from other systems, such as B2C Commerce or Service Cloud, to trigger or personalize journeys. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_jb_journey_builder.htm&type=5

A system is a software application or platform that provides specific functionality or capabilities for a business. A system can be either internal or external to Salesforce, depending on where it is hosted and how it is accessed. To deliver key capabilities for its B2C business, a company should use the following systems:
Sales Cloud: Sales Cloud is a product that allows managing sales processes and activities across different channels and devices. Sales Cloud can help the company to deliver a full list of all customer orders throughout their engagement lifetime, by integrating with B2C Commerce and other systems to capture and display order information on customer records.
Marketing Cloud: Marketing Cloud is a product that allows creating and delivering personalized marketing campaigns across different channels and devices. Marketing Cloud can help the company to provide lifetime engagement tracking and history of the customer, by integrating with B2C Commerce and other systems to collect and analyze customer behavior and interactions across touchpoints.
Salesforce Order Management: Salesforce Order Management is a product that allows managing and fulfilling orders across different channels and systems. Salesforce Order Management can help the company to calculate the lifetime value of customers based on their orders, by integrating with B2C Commerce and other systems to aggregate and report on order revenue and profitability.
Option B is incorrect because Service Cloud is a product that allows managing customer service interactions across different channels and systems. Service Cloud does not provide any of the capabilities that the company needs, such as delivering a full list of customer orders, providing lifetime engagement tracking, or calculating lifetime value of customers. Option C is incorrect because Heroku is a platform that allows building, running, and scaling custom web applications using various languages and frameworks. Heroku does not provide any of the capabilities that the company needs, such as delivering a full list of customer orders, providing lifetime engagement tracking, or calculating lifetime value of customers. Option D is incorrect because Materials Written is not a system, but rather a term that refers to written documents or content that are used for communication or education purposes. Materials Written does not provide any of the capabilities that the company needs, such as delivering a full list of customer orders, providing lifetime engagement tracking, or calculating lifetime value of customers. Reference:
https://www.salesforce.com/products/sales-cloud/overview/
https://www.salesforce.com/products/marketing-cloud/overview/
https://www.salesforce.com/products/commerce-cloud/ecommerce/order-management/