If you mean that you would like each html tag to contain the id, you could do it using the option's value attribute, or alternatively, by using html5 data attributes:
{{#each case}}
<option value="{{_id}}" data-id="{{_id}}">{{name}}</option> // can use value= or data-id=
{{/each}}
This will be visible in the browser's source code, but not on screen to the user. Alternatively, if you want to access the id through template events, it will be available there as well:
"click option" /*or whatever event handler is appropriate here*/: function(event, template){
var id = template.data._id;
/* alternatively you can call this._id which will generally give you the same result */
/* then do your event handling, etc. */
}