An AppExchange solution is a pre-built application or component that can be installed from the Salesforce AppExchange, which is an online marketplace for Salesforce products. AppExchange solutions can help you meet your business needs quickly and efficiently, without requiring much development effort or expertise. You can browse and search for AppExchange solutions by category, industry, rating, price, and more.

A validation rule is a formula that evaluates the data in one or more fields and returns a value of "True" or "False". Validation rules verify that the data a user enters in a record meets the standards you specify before the user can save the record. In this case, a validation rule can be used to check if the Expected_Close_Date_c field is populated before the Opportunity stage is set to qualified

The deployed permission set will merge with the existing permission set in production, meaning changes made in the sandbox will be added to the existing permission set after deployment.
Reference:
Salesforce Help - Permission Sets

The app builder should go to the Account object to build these new rollups. A roll-up summary field is a type of field that can aggregate numeric values from child records related to a parent record by a master-detail relationship. The app builder can create two roll-up summary fields on the Account object that count the number of open Opportunities and the number of won Opportunities related to each Account. Lightning App Builder is not a place where the app builder can build these new rollups. Lightning App Builder is a tool that allows the app builder to customize the layout and components of Lightning pages, such as record pages, home pages, or app pages. It cannot create new fields or relationships on objects. Lightning Object Creator is not a place where the app builder can build these new rollups. Lightning Object Creator is a tool that allows the app builder to create custom objects from spreadsheet data by uploading a file and mapping columns to fields. It cannot create roll-up summary fields or relationships on existing objects. Opportunity object is not a place where the app builder can build these new rollups. The Opportunity object is the child object in the relationship with the Account object, and roll-up summary fields can only be created on the parent object.

To test code against a subset of production data under 5 GB and to refresh the sandbox environment weekly, the most suitable type of sandbox is:
Partial Copy (B). A Partial Copy sandbox includes a copy of your production org's metadata and a subset of your production data as defined by the sandbox template. It allows up to 5 GB of data and can be refreshed every 5 days, making it ideal for scenarios where a representative sample of production data is needed for testing without the storage and refresh limitations of a Full Copy sandbox.
Developer Pro (A) and Developer (D) sandboxes also allow for testing and development but do not include a subset of production data-they either have no data or only limited sample data. A Full Copy sandbox (C) provides a full replica of production data, far exceeding the 5 GB requirement and generally is refreshed less frequently due to larger data volume and longer copy times.
Reference for sandbox types and their capabilities:
Sandbox Types and Templates: https://help.salesforce.com/articleView?id=sf.data_sandbox_implementation_tips.htm&type=5