Create two drop down fields "State" and "City". When state is selected, appropriate cities under that state should be displayed. Ex: States - Telangana, Andhra Pradesh, TamilNadu etc..

[d v sumanth1]

thats it

[Deepak Kumar5]

You can set "State" field in dependent for City. In Choice list, set state list first then for city, add state choices in dependent value.

Refer Incident's category and sub-category fields.

[Dan Shores · Accepted solution]

Deepak's suggestion to use a dependent field is definitely the best option, if you have issues a second option could be a client script using g_form.getValue('field') and using an if statement with g_form.addOption('city', 'option value') or g_form.removeOption('city', 'option value') within. 

Below is an example of an onload client script i recently used to remove an option on a specific change type, for some reason dependent values were not working in this case (have used them elsewhere so not sure why this was different).

function onLoad() {
    //Type appropriate comment here, and begin script below
    var type = g_form.getValue('type');
    var impact = g_form.getValue('u_moc_level_of_impact');

    if (impact ==  'critical' && type != 'moc_global') {
		g_form.addErrorMessage('Level of impact can no longer be critical on Non-Global MOC records. Impact has been changed to Major on this MOC.');
        g_form.setValue('u_moc_level_of_impact', 'major');
		g_form.removeOption('u_moc_level_of_impact', 'critical');
    } if(type != 'moc_global'){
		g_form.removeOption('u_moc_level_of_impact', 'critical');
		
	}
   
}

Thanks,

Dan