Edit in JSFiddle

$(document).ready(function(){
    $('#btnSetSelectedIndex').click(function(){
       $("#ddlList").get(0).selectedIndex = 1;
    });
    
     $('#btnSetByValue').click(function(){
       $("#ddlList").val(3);
    });
       
    $('#btnSetByText').click(function(){
        $('#ddlList option:contains("HTML")').attr("selected","selected");
    });
});

<select id="ddlList">
  <option value="1">ASP.NET</option>
  <option value="2">C#</option>
  <option value="3">VB.NET</option>
  <option value="4">HTML</option>
  <option value="5">jQuery</option>
</select>

<br/><br/>
<input type="button" id="btnSetSelectedIndex" Value="Set Selected Index" />
&nbsp;&nbsp;
<input type="button" id="btnSetByValue" Value="Set By Value" />
&nbsp;&nbsp;
<input type="button" id="btnSetByText" Value="Set By Text" /> 
<br/><br/>