You have to handle the 'change' event of the select element in jQuery and do the stuff you want through ajax calls basically.
$('#ddlCountry').change(function() {
});
Use jQuery method .ajax() to populate the state dropdown here. Hope the below cod eexample will definitely of help for you :-
Ex
$.ajax({
type: "POST",
url: "../WebService.asmx/GetAllStates",
data: "{CountryID:'" + $('#ddlCountry').val() + "'}",
contentType: "application/json; charset=utf-8",
dataType: "json",
async: false,
success: function (response) {
$.each(JSON.parse(response.d), function (key, value) {
$("#ddlState").append("<option value='" + value.StateID + "'>" + value.StateName + "</option>");
});
},
error: function (XMLHttpRequest, textStatus, errorThrown) {
alert(failMessage);
}
});