JIT (Just-in-Time) compilation in Angular compiles the application's TypeScript code into JavaScript at runtime, whereas AOT (Ahead-of-Time) compilation compiles the code during the build process, producing optimized and smaller JavaScript bundles, which results in faster loading times and better performance.

For a directive to modify the value of a host property, you would use the HostBinding decorator. HostBinding allows you to bind the value of a directive property to a host element's property, effectively modifying the host element's behavior or appearance. This is useful for creating dynamic and interactive custom directives in Angular.

The term "Hierarchical Injector" in Angular refers to a type of dependency injection system that supports multiple hierarchies. In Angular, services can be provided at various levels of the application, and the hierarchical injector helps manage the scope and accessibility of these services. This is crucial for controlling how services are shared and instantiated within different parts of an Angular application.

The main purpose of Angular's ChangeDetectionStrategy.OnPush is to improve change detection performance. When you use this strategy, Angular will only perform change detection when input properties of a component change or when certain events are triggered, leading to better performance in applications with complex component trees. The other options are not the main purpose of this strategy.

ViewContainerRef in Angular is used to manage the DOM elements where dynamic components are inserted. It provides a reference to the container where you can dynamically add or remove components. This is useful for scenarios where you need to render components based on user interactions or application logic. The other options are not the primary purpose of ViewContainerRef.

To restrict loading of modules based on user roles in a lazy-loaded scenario, you should implement the CanLoad guard. It allows you to check conditions before the module is loaded, ensuring that only authorized users can access certain parts of the application.

To send a GET request using HttpClient, you would use the get() method. This method is used to perform HTTP GET requests to retrieve data from a specified URL.

In Angular's routing, the CanLoad guard determines whether a module can be lazily loaded. This guard is used to protect routes that are loaded on-demand, typically through lazy loading, and it checks conditions before allowing the module to load. CanActivate is used to determine if a route can be activated, but it doesn't control lazy loading.

In RxJS, the forkJoin operator is used when you need to combine multiple Observables and wait for all of them to complete before emitting a result. It will emit an array of the last values from each Observable when they have all completed.

To implement an undo feature in NgRx, you would leverage "Time-travel debugging." This feature allows you to step backward and forward through the state changes, effectively enabling undo and redo functionality in your application.