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<form [formGroup]="form">
  <input type="checkbox" formControlName="optionA"> Option A
  <input type="checkbox" formControlName="optionB"> Option B
  <input formControlName="optionBExtra" placeholder="Reason" *ngIf="optionBExtra">
</form>

export class AppComponent {
  form: FormGroup;

  ngOnInit() {
    this.form = new FormGroup({
      optionA: new FormControl(false),
      optionB: new FormControl(false),
      optionBExtra: new FormControl('', [ Validators.required ])
    });
  }

  submit() {
    if(this.form.valid) { ... }
  }
}