The app builder should do three things to optimize mobile experience:
Use Global Actions to make it easy to perform vital functionality on mobile: Global actions are actions that allow users to create records or perform tasks from anywhere in Salesforce. They are displayed in various places in Salesforce mobile app, such as action bar, action menu, or publisher. By using global actions, the app builder can make it easy for the appraisal team to perform common tasks on mobile, such as creating a new appraisal record or logging a call.
Minimize the amount of formula fields and lookup fields to reduce page load time: Formula fields and lookup fields are types of fields that can affect the performance of Salesforce mobile app. Formula fields are fields that calculate a value based on an expression or formula. Lookup fields are fields that create a relationship between two objects and allow users to select a value from a list of records. Both types of fields can increase the page load time on mobile devices, especially if they are complex or reference many other fields. By minimizing the amount of formula fields and lookup fields, the app builder can optimize the mobile experience for the appraisal team.
Put the most important fields in the compact layout so they are easy to find: Compact layouts are layouts that display a record's key information at a glance. They are used in various places in Salesforce mobile app, such as record highlights, list views, related lists, and lookup dialogs. By putting the most important fields in the compact layout, the app builder can ensure that the appraisal team can easily find the information they need on mobile devices. Option B and D are not things that an app builder should do to optimize mobile experience.

The functionality that should be used to set up packages is namespace. A namespace is a unique identifier that distinguishes one package from another and prevents naming conflicts with other packages or customizations. A namespace also allows developers to expose their code or components for use by other developers through APIs or web services. A description is not a functionality that should be used to set up packages, but a text field that provides information about the package contents or purpose. A description does not prevent conflicts with other packages or customizations. Allow sharing is not a functionality that should be used to set up packages, but a setting that controls whether users can share records owned by other users within their role hierarchy. Allow sharing does not prevent conflicts with other packages or customizations. Help setting is not a functionality that should be used to set up packages, but a setting that determines whether users can access help documentation for standard Salesforce features or custom features within a package. Help setting does not prevent conflicts with other packages or customizations.

To facilitate quick managerial sign-off on discounted opportunities:
B . Dynamic Actions: Allows the display of specific actions on the Opportunity record page based on certain conditions, like the discount percentage.
D . Approval Process: Configures a formal sign-off process for records meeting specific criteria, such as discount ranges between 20% to 30%.
Steps to set up:
Configure an approval process for opportunities where the discount is between 20% and 30%.
Use Dynamic Actions to conditionally show the approval action directly on the Opportunity record page based on the discount percentage.
This setup allows sales managers to efficiently review and approve discounted deals directly from the Opportunity record page.
For guidance on setting up approval processes and dynamic actions, refer to Dynamic Actions in Salesforce.

AppExchange is a marketplace where app builders can find prebuilt resources to help with migration and adoption projects. AppExchange offers apps, components, solutions, and consulting services that can assist with moving from Classic to Lightning Experience. Import Wizard, Lightning Object Creator, and Flow Builder are not sources of prebuilt resources, but tools for data import, object creation, and automation respectively.

To manage frequently changing picklist values effectively:
B . Set up Global Values in Picklist Value Sets. This approach allows for centralized management of picklist values that can be reused across multiple fields and objects. By using global value sets, inactive values can be efficiently managed and deactivated without impacting fields that use these sets.
For further information on managing picklists, check Salesforce's Picklist Management guide.