export class Question {
    constructor(
        public Id: number,
        public Title: string,
        public Text: string,
        public OriginalUrl: string,
        public Answers: Answer[]
    ){}

    isAnsweredCorrectly():boolean {

        for(let answer of this.Answers){
            if(
                (!answer.IsCorrect && answer.IsSelected) ||
                (answer.IsCorrect && !answer.IsSelected)) {

                return false;
            }
        }

        return true;
    }
}

<ul class="list-group h-100">

    <a class="list-group-item list-group-item-action"
       *ngFor="let q of questions; let i = index;"
       [ngClass]="getClasses(q)"
       (click)="selectQuestion(q)">
       Question {{i+1}}
    </a>

</ul>

export class QuestionListComponent implements OnInit {

  @Input() questions:Question[];
  @Input() currentQuestion:Question;

  ...

  getClasses(question:Question):any{
    return {
      'list-group-item-primary': question == this.currentQuestion,
      'list-group-item-success': question.isAnsweredCorrectly(),
      'list-group-item-danger': !question.isAnsweredCorrectly()
    };
  }
}

QuestionListComponent.html:3 ERROR TypeError: question.isAnsweredCorrectly is not a function
    at QuestionListComponent.getClasses (question-list.component.ts:33)
    at Object.eval [as updateDirectives] (QuestionListComponent.html:5)