Sharing rules and manual sharing are two features that can extend record access beyond the organization-wide defaults. Sharing rules are automatic rules that grant access to groups of users based on certain criteria, such as role, public group, or territory. Manual sharing is a manual way of granting access to individual users or groups by the record owner or someone above them in the role hierarchy. Option B and C are not features that extend record access.

When activating a Lightning page from the Lightning App Builder, three options are available:
Assign the page to a combination of apps and profiles. This option allows users to specify which apps and profiles will see the custom Lightning page instead of the default page.
Make the page the org default. This option makes the custom Lightning page the default for all users in the org regardless of their app or profile.
Make the page the default homepage for specific apps. This option allows users to set different homepages for different apps in Lightning Experience. [Source]

If the app builder tries to convert a preexisting long text area field to text area, preexisting information will truncate to the first 255 characters. This means that any information beyond 255 characters will be lost. Option B is incorrect because preexisting information will not remain if it was over 255 characters. Option C is incorrect because preexisting information will not cause an error message to pop up. Option D is incorrect because preexisting information in the field will not be completely lost, only truncated.

Schema Builder allows users to create custom objects, fields, and relationships using a drag-and-drop interface. However, one of the limitations of Schema Builder is that fields and relationships can be created, but they will be unable to add the fields to the page layout from the canvas. Users will need to go to the Lightning Object Manager or Setup to edit the page layout and add the fields.

To restrict visibility of the Jupiter field on the Solar System object specifically to users with the Outer Planets profile:
C . Field-level security (FLS). This allows the app builder to control access to fields based on user profiles.
Steps to set up field-level security:
Navigate to Setup → Object Manager → Solar System.
Select Fields & Relationships → Jupiter field.
Click Field-Level Security.
For all profiles except Outer Planets, set the Visible option to unchecked, ensuring these users cannot see the field.
For the Outer Planets profile, check Visible to grant access.
This configuration ensures that only users with the Outer Planets profile can view the Jupiter field.
For more information on field-level security, check Salesforce's guide on Field-Level Security.