Explanation
A validation rule is a tool that allows administrators to enforce data quality and integrity by preventing users from saving records that do not meet certain criteria or conditions. For example, a validation rule can display an error message if a user enters an invalid value in a field. In this case, the administrator can create a validation rule on the account object that displays an error if the account annual revenue field is below 0 or greater than 100 billion. References:
https://help.salesforce.com/s/articleView?id=sf.validation_rules.htm&type=5

Explanation
A Trailhead Playground is a free, online learning environment that allows you to try out Salesforce features and apps. You can use a Trailhead Playground to test out an app from the AppExchange before you install it in your production org.
A sandbox is a copy of your production org that you can use to test changes and new features. You can install an app from the AppExchange in a sandbox to see how it works in your environment.
Testing an app in a production org is not recommended, as it could affect your live data. Checking edition compatibility is important, but it is not a way to try out an app.

Explanation
Report type is a tool that can be used to pullcontact fields into the custom report for Event. Report type defines the set of records and fields available to a report based on the relationships between a primary object and its related objects. To edit the custom Event report type and add fields related via lookup, go to Setup > Report Types and select the Event report type. Then click Edit Layout and drag the fields from the Contact object to the layout.
References:https://help.salesforce.com/s/articleView?id=sf.reports_builder_create_report_type.htm&type=5