The best license type to fulfil US's requirements is the Service Cloud license. Service Cloud licenses are designed for users who need access to customer service features, such as cases, solutions, knowledge articles, entitlements, service contracts, and service console. Service Cloud users can also access standard CRM objects, such as accounts, contacts, leads, opportunities, campaigns, and reports3. Lightning Platform Starter license is not a good option because it is intended for users who need access to one custom app and a limited set of standard objects. Salesforce license is not a specific license type, but rather a generic term for any license that grants access to the Salesforce platform. Sales Cloud license is not a good option because it is intended for users who need access to sales features, such as products, price books, quotes, orders, and forecasts.

Leveraging duplicate rules in Salesforce to validate duplicates during the account creation process (option A) is the best option to implement to avoid duplicates, as it allows the sales associates to identify and merge duplicate accounts before they are saved. Developing an Apex class that searches for duplicates and removes them nightly (option B) is not a good option, as it may cause data loss or conflicts, and it does not prevent duplicates from being created in the first place. Implementing an MDM solution to validate the customer information before creating Salesforce (option C) is also not a good option, as it may introduce additional complexity and cost, and it does not address the issue of customers interacting with both the website and the store. Building a custom search functionality that allows sales associates to search for customer in real time upon visiting their retail stores (option D) is also not a good option, as it may not be reliable or user-friendly, and it does not leverage the existing Global Search feature.