The Highlights Panel in the Lightning Record Page displays key fields and record actions prominently at the top of the page, exactly as described in the requirement.
Why A (Highlights Panel) is correct:Exact Extract:"The highlights panel displays important fields and key record actions at the top of the record page in Lightning Experience." - Salesforce Help | Highlights Panel Why not B:Path is used to visualize progress through stages (e.g., Opportunity Stage), not display multiple fields.
Why not C:A custom component is unnecessary since the standard Highlights Panel fulfills the requirement.
Why not D:Accordion is used to organize information in collapsible sections, not display key fields at the top.
Reference:
Salesforce Help | Highlights Panel
Salesforce Platform App Builder Exam Guide | User Interface

The best way to configure Account sharing so that community users only see their own Account is to set the account external OWD to private. This will restrict the access to Account records for external users based on the sharing rules and criteria. Creating an account record type, defining an owner-based sharing rule, or defining a permission set will not prevent community users from seeing other accounts.

Two reasons that can prevent an app builder from adding a custom component to the Lightning home page are if My Domain is not deployed, or if the component is tagged for record pages instead of home pages. My Domain must be deployed to add custom components to the page with the App Builder, as it enables Lightning components and other features. The component must be tagged for home pages in its configuration file, otherwise it will not show up in the App Builder for home pages

The three reasons user A has access but user B is unable to access the record are:
User A is on the same account team as user C. Account teams are groups of users who work together on an account. Users who are on an account team can have access to the account and related records owned by other team members.
User C has manually shared the record with user A. Manual sharing is a way of granting access to individual users or groups by the record owner or someone above them in the role hierarchy.
User A is in a public group that has access via a sharing rule. Public groups are groups of users who can be used to share data or assign permissions. Sharing rules are automatic rules that grant access to groups of users based on certain criteria. Option B and D are not reasons for user A's access.

Upon adding a lookup field to Account from a custom object, Box:
D . Accounts with Boxes. Salesforce automatically creates a new report type that reflects the relationship from the perspective of the parent object, in this case, Accounts, showing related Box records.
This new report type enables users to create reports that list Accounts along with their associated Boxes, if any.
For further understanding of report types and relationships, Salesforce's guide on Report Types provides comprehensive information on how these are structured following modifications to object relationships.