how to add onchange event on dropdown in angular ?

Use (change) event instead of (ngModelChange).

<select class='select-option'
    #mySelect
    (change)='onOptionsSelected(mySelect.value)'>
   <option class='option' 
   *ngFor='let option of dropDownData' 
   [value]="option.seo_val">{{option.text_val}}</option>
</select>

In typescript file:

onOptionsSelected(value:string){
     console.log("the selected value is " + value);
}

try this.

<select class='select-option' [(ngModel)]="selected"
     (change)='onOptionsSelected($event)'>
   <option class='option'  *ngFor='let option of dropDownData' 
   [value]="option.seo_val">{{option.text_val}}</option>
</select>

public onOptionsSelected(event) {
   const value = event.target.value;
   this.selected = value;
   console.log(value);
}