Angular 4 setting selected option in Dropdown

If you want to select a value based on true / false use

[selected]="opt.selected == true"

for example:

<option *ngFor="let opt of question.options" 
    [value]="opt.key" 
    [selected]="opt.selected == true">
        {{opt.selected+opt.value}} 
</option>

checkit out

Angular 2 – Setting selected value on dropdown list