The correct answer is "JoinLeft". In LINQ, to perform a left outer join, you need to use the Join method along with the DefaultIfEmpty method. This combination allows you to include results from the left side of the join even if there are no matches on the right side.

LINQ queries with lambda expressions and aggregations are often preferred in scenarios involving filtering and aggregating data from multiple sources due to their concise syntax and ability to handle complex logic effectively. Lambda expressions provide flexibility in defining custom conditions and aggregations, making them suitable for diverse data manipulation tasks.

The impact of using Include and ThenInclude methods in LINQ queries is decreased performance due to eager loading of related entities. Eager loading retrieves all related entities in a single query, which can lead to increased data transfer and processing overhead, resulting in slower performance compared to lazy loading.

Performance optimization in LINQ queries with large data sets can be achieved by using pagination and filtering techniques. Pagination involves retrieving data in smaller chunks or pages, reducing the amount of data transferred and processed at once. Filtering techniques allow for narrowing down the data set to only the relevant records, further improving query performance.

The correct answer is "GroupBy". The GroupBy method in LINQ is used to group query results based on a specified key. It allows you to perform operations on each group separately, such as aggregation or filtering. This is commonly used for grouping data in LINQ queries.

The correct answer is "Sum". In LINQ, the Sum extension method is used for aggregating numeric data. It calculates the sum of all the values in a collection or the sum of a specific property of elements in the collection. This is commonly used for calculating totals or subtotals.

Writing efficient LINQ queries involves minimizing unnecessary data retrieval from the database. Projections allow you to select only the required fields, reducing the amount of data transferred over the network and improving query performance. By fetching only essential data, you can optimize resource utilization and minimize database load, especially in scenarios with complex data models.

LINQ's Select method enables the transformation of query results into a custom data structure by specifying the desired fields or properties. By projecting query results, you can shape the output according to the requirements, facilitating seamless integration with downstream processing or presentation layers. This approach enhances code readability and maintainability by encapsulating the data transformation logic within the LINQ query.

A projection query in Entity Framework involves selecting specific columns from a table rather than retrieving all columns. It helps in reducing data transfer and can improve performance, especially when dealing with large datasets.

In LINQ and Entity Framework, the Select method is commonly used to specify which columns to retrieve from a table. It allows developers to project only the required data, leading to more efficient queries and reduced overhead.