A primary key in Entity Framework serves to uniquely identify each record in a table. It ensures that each row in the table can be uniquely identified. This is crucial for data integrity and efficient data retrieval operations. Without a primary key, it would be challenging to manage and manipulate data effectively.

The correct option is Option 1: NotMapped. This data annotation is used to specify that a property should be excluded from the database schema. It is typically applied to properties that are not mapped to any database table column, such as calculated properties or properties used for internal processing within the application. Using NotMapped prevents Entity Framework from attempting to create a corresponding column in the database.

Entity Framework supports several strategies for implementing complex inheritance structures. One common approach is Table Per Hierarchy (TPH), where all classes in the inheritance hierarchy are mapped to a single table with a discriminator column to distinguish between different types. This simplifies the database schema but can lead to unused columns and potential performance issues.

In Entity Framework, a composite key is defined by using the [Key] attribute on multiple properties within an entity class. This tells Entity Framework that the combination of these properties forms a unique identifier for each record in the table. Composite keys are useful when a single property cannot uniquely identify records, and a combination of properties is needed for this purpose.

Indexes in Entity Framework are primarily used to improve query performance. By creating indexes on columns frequently used in search criteria or join conditions, the database management system can locate the relevant rows more efficiently, leading to faster query execution. However, it's essential to use indexes judiciously as they can also impact insert and update performance.

Entity Framework automatically creates indexes on foreign key columns to optimize performance. These indexes help in efficient querying and joining of related entities, resulting in faster database operations.

Not defining an index on a frequently queried column can lead to slower query performance as the database engine needs to perform full table scans to locate the desired data. This results in increased overhead and longer query execution times, impacting the overall performance of the application.

When mapping an Entity Type to a database view in Entity Framework, you can use the [Table] attribute on the entity class and provide the database view name as a parameter. This approach explicitly specifies the mapping between the entity and the database view. This ensures that Entity Framework knows how to interact with the database view when querying or modifying data.

In Entity Framework, you can specify an index to be unique by setting the IsUnique property to true within the OnModelCreating method of the DbContext class. This configuration ensures that the corresponding index in the underlying database is unique, preventing duplicate entries in the indexed column.

Index fragmentation in Entity Framework can degrade query performance by causing unnecessary disk I/O operations. Fragmented indexes require additional disk reads to locate and retrieve data, which can significantly slow down query execution, especially for large datasets.