The potential reason for the valueChanges observable not emitting values when the custom form control's value changes could be that the custom form control is missing the FormControlName directive in the template. This directive binds the control to the form and allows it to propagate changes. Options a and b are not the typical reasons for this issue, and option c is unlikely unless there are other symptoms of a deprecated version. It's crucial to ensure that the custom form control is correctly integrated into the Angular form template.

To start the execution of an Observable and receive values from it, you need to call the subscribe method on the Observable instance. This method takes one or more callback functions to handle emitted values, errors, and completion.

In ViewEncapsulation.None mode, styles defined in the component can affect all elements in the application, and global styles can affect the component's elements. This mode effectively removes the shadow DOM boundary and makes the component's styles global. The other options describe the behavior of different encapsulation modes (such as ViewEncapsulation.Emulated, ViewEncapsulation.ShadowDom, and ViewEncapsulation.Native) but not the specific behavior of ViewEncapsulation.None.

In Angular, when using the OnPush change detection strategy, a component will be checked for updates not only when its input properties change but also when events triggered by user interactions occur. This is because user interactions can lead to changes in a component's view, and OnPush checks for such changes to provide more efficient and responsive updates.

To handle complex state transitions in Ngxs (a state management library for Angular), one would utilize "Effects." Effects are used to manage side effects, such as HTTP requests, that can change the state in response to dispatched actions.

When creating a service in Angular using the Angular CLI, you would use the command ng generate service. This command generates the necessary files and boilerplate code for an Angular service, making it a convenient way to create services in an Angular project. The other options (ng serve, ng create service, ng build service) are not the correct commands for creating a service.

In Angular, the ngAfterViewInit lifecycle hook is most appropriate when you need to perform initialization logic after a component's views have been rendered. This hook is called once when the component's view and its children's views are initialized. ngOnInit is called earlier, before child views are ready, and the other options are not typically used for initialization logic in this context.

Angular's default change detection strategy checks components at predefined intervals. It periodically scans all components in the application for changes, which can be resource-intensive. This approach is less efficient than the OnPush strategy because it checks components even when no relevant changes have occurred. User interaction and asynchronous data arrival do not directly trigger checks in the default strategy.

In Angular, it's best to use the ngOnInit lifecycle hook to handle any additional initialization tasks, including making HTTP requests. The constructor is mainly used for dependency injection and not for logic.

In Angular's router, the pathMatch property is significant because it specifies how to match a URL path to a route. It has two values: 'prefix' (default) and 'full.' 'Prefix' means the route will match if the URL starts with the path, while 'full' requires an exact URL match. The other options do not accurately describe the purpose of pathMatch.