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);
}