The correct answers are C. Unique and D. External ID.
The legacy ID should be stored in a custom field marked as an External ID because Salesforce uses External ID fields to identify records from an outside system and to support update/upsert operations during future data loads. The field should also be marked Unique so each legacy system ID maps to only one Salesforce Account record and avoids duplicate-match problems during upsert. Salesforce documentation defines External ID fields as fields that contain record identifiers from outside Salesforce and can be used to update or upsert records.
A . Required is not necessary for the upsert-matching requirement.
B . Encrypted/Text encrypted is not appropriate because encrypted fields are not used for this legacy-ID matching purpose.

The correct answer is A. Existing reports created under the Sun__c with Planet__c report type made by Salesforce will be unusable, but not deleted.
When a relationship is changed from master-detail to lookup, the available standard report types can change. Salesforce documentation notes that converting a lookup relationship to master-detail or vice versa can make existing custom reports unusable because of differences in available standard report types.
B is incorrect because roll-up summary fields are not automatically deleted from reports as part of the conversion. In fact, Salesforce states that a master-detail relationship cannot be converted to lookup while roll-up summary fields exist on the master object.
C is incorrect because roll-up summary fields do not remain and simply stop updating.
D is incorrect because the reports are not deleted; they become unusable.

The correct answer is D. Rich text area.
The requirement says users need to write detailed descriptions that can include pictures and links. Salesforce Rich Text Area fields are designed for formatted text and support images and hyperlinks.
Long text area supports longer plain text but not rich formatting with images. URL stores a single web address. Multi-select picklist stores selected values, not detailed formatted descriptions.

The correct answers are A. Set up new public Chatter groups and D. Use post action on the Account object.
Public Chatter groups support open collaboration where company users can share information in a group feed. Salesforce Chatter groups are designed for teams to share information, files, and discussions.
Post action on the Account object allows users to discuss customer accounts directly from the Account record feed. Salesforce Chatter feeds appear on record pages and allow users to follow records and see updates about them.
Why others are incorrect:
Private Chatter groups are not suitable because CK wants all company employees to see the conversations.
Chatter actions to create tasks help assign work, but they do not primarily solve open company-wide collaboration around accounts.

The correct answer is A. Partial Copy.
A sandbox template is used to define which object data is copied into a sandbox. Salesforce documentation states that sandbox templates control what data is copied into Full or Partial Copy sandboxes. Since the question says the app builder created a sandbox template for Accounts, Projects, and Project Milestones, the correct testing environment is Partial Copy because it includes metadata plus a selected sample of production data defined by the sandbox template. Developer, Developer Pro, and Scratch Orgs do not use sandbox templates for selected production data in this way.