The app builder should use a custom button and Flow to achieve the desired result. A custom button is a button that can invoke custom logic or actions when clicked by a user. A Flow is a type of automation tool that can guide users through a series of screens and perform actions based on user input. In this case, the app builder can create a custom button on the Account object that launches a Flow when clicked by a user. The Flow can display a popup window with questions for the user to answer and populate additional fields on the Account record based on the user input4. Option A, B, and C are not tools that can achieve this result.

The feature that the app builder should use to allow users to create the opportunity is a quick action. A quick action is a type of custom button or link that can create or update records, log calls, send emails, or launch flows from a record page or a global menu. The app builder can create a quick action on the account object that creates an opportunity record with pre-populated fields. Option B is incorrect because a default action is not a valid term, but rather a standard action that appears on every record page by default, such as Edit or Delete. Option C is incorrect because a custom button is not suitable for this requirement, as custom buttons can only display a URL or execute JavaScript when clicked. Option D is incorrect because a custom link is not suitable for this requirement, as custom links can only display a URL or execute JavaScript when clicked.

Dynamic actions are a new feature that allow app builders to create custom buttons or actions that can call Lightning Web Components and display them conditionally based on field values or other criteria. Formula fields, AppExchange products, and custom links are not able to call Lightning Web Components.

Workflow rule and an outbound message or Process Builder and Apex Code are two ways to integrate Salesforce with an external web service. Workflow rule can define criteria and actions for triggering an outbound message, which is an XML message sent to a designated endpoint URL. Process Builder can invoke Apex Code, which is a programming language that can make callouts to external web services using HTTP requests. Flow Builder and an outbound message or Process Builder and an outbound message are not possible combinations.

Apex. This is correct because Apex is a programming language that can perform complex logic and manipulate data in Salesforce. Apex can be used to create a trigger on the opportunity object that copies the value of the billing state from the related account record when the opportunity is created, and prevents it from being changed afterwards. The other options are not suitable for this requirement because they either cannot copy data from a related object (formula field, roll-up summary field) or cannot prevent data from being changed (workflow).