Sharing rules can be used to extend access to records owned by certain users or roles to other users or roles. In this case, a sharing rule can grant read-only or read/write access to sales-owned accounts to the marketing team

The validation rule should be AND(ISNEW(), ISBLANK(Lead.Owner:User.Id)). This rule will prevent saving a new lead record if the owner is not a user. This will ensure that campaign members have a sales owner. Option A is incorrect because it will prevent saving any lead record if the owner is blank, which is not the requirement. Option B is incorrect because it will prevent saving an existing lead record if the owner is a queue, which is not the requirement. Option D is incorrect because it will prevent saving any lead record if the owner is a queue, which is not the requirement.

Partial Copy Sandboxes are ideal for testing and development purposes as they include a subset of production data and can be refreshed more frequently than Full Copy sandboxes.
Reference:
Salesforce Help - Types of Sandboxes

TEXT() is a formula function that converts a picklist value to text. This can be used to display the Type field from the parent object Galaxy on the child record Star. ISPICKVAL() is used to check if a picklist value is equal to a specified value. VALUE() is used to convert text that represents a number to a number. FIND() is used to locate one text string within another text string.

The best actions to take to avoid creating duplicate leads when uploading a list of 400 leads are to utilize a Lead Matching Rule and corresponding Duplicate Rule to block newly created duplicate leads, and to upload the lead list using the import wizard and select a Matching type to prevent duplicate lead creation. The Lead Matching Rule defines the criteria for identifying duplicate leads, and the Duplicate Rule defines the actions to take when a duplicate is detected. The import wizard allows users to choose a Matching type based on the Lead Matching Rule and block or allow duplicates. Using Data Loader's update function or enabling Duplicate Matching in the Data Management section will not prevent duplicate lead creation.