The correct answers are A. Unique and D. External ID.
The legacy ID should be marked as an External ID because Salesforce can use an external ID field to match records during import, integration, and upsert operations. Salesforce REST API documentation states that records can be inserted or updated using a specified external ID field.
The field should also be marked Unique so that the legacy system ID identifies one Salesforce Account record only. Salesforce documentation explains that an external ID can represent an ID from another external system, and uniqueness helps prevent duplicate matching values.
B is incorrect because encrypted text is not appropriate for a matching/upsert key.
C is incorrect because Required only forces a value; it does not make the field usable for upsert matching or prevent duplicate legacy IDs.

A schedule-triggered flow is designed for batch automation that must execute at a defined time and frequency.
Salesforce allows this flow type to run once, daily, or weekly and to select an object plus entry conditions in the Start element. Universal Containers therefore can configure Account as the target object, Weekly as the frequency, Sunday at 2:00 a.m. as the start schedule, and conditions that identify Accounts whose Last Activity Date is more than 90 days old. A scheduled path on a record-triggered flow is tied to individual record-triggering events rather than a weekly evaluation of all qualifying Accounts. Option D runs only once and fails the recurring requirement. Salesforce explicitly supports weekly schedule-triggered flows for this batch-processing pattern.

The correct answer is B. Add the standard Chatter feed component.
In Lightning App Builder, Salesforce provides a standard Chatter component. Salesforce documentation states that the Feed component was renamed Chatter in Lightning App Builder and is used to place the publisher and feed on a record page.
A is incorrect because a custom Chatter feed component is not needed.
C is incorrect because this does not come from the AppExchange/App College.
D is incorrect because Chatter feed is not added by using the standard related list component.

The correct answer is B. Custom action.
The user wants to edit a record's Status and enter a custom Due Date quickly from the record feed in the Salesforce mobile app. A custom action, specifically an object-specific quick action, can expose selected fields for fast updates on mobile. Salesforce documentation states that object-specific actions let users quickly create or update records in the context of a specific record, and the Salesforce mobile action bar displays actions in mobile places such as feeds and record pages.
Custom button and URL formula field are not the best mobile feed-editing tools. Custom quick access link is not the correct Salesforce feature for this requirement.

To optimize Salesforce Mobile experience:
* B. Minimize formula fields and lookup fields improves page load performance on mobile devices.
* C. Put key fields in the compact layout ensures important information is visible immediately.
* E. Use Global Actions simplifies creating or updating records from mobile quickly.
Salesforce documentation on mobile best practices recommends limiting complex fields, prioritizing compact layouts, and using quick actions for essential functionality. (Salesforce Help)
* A is incorrect because default values improve speed and reduce user errors, not eliminate them.
* D is unnecessary; Lightning layouts are responsive and do not require separate OS-specific layouts.