[SalesForce] How to disable an ‘option’ from ‘lightning:select’

I'm pretty sure you just call component.getLocalId()

So it'd be:

var a = event.getSource();
var id = a.getLocalId();

To find the local ID for a component in JavaScript, use cmp.getLocalId().

Source here:

Setting the options doesn't actually cause the rendering to happen. So, the value you're setting is ignored because it isn't valid at the time. The solution is to use another callback post-rendering. Here's an example app I wrote that demonstrates this:

<aura:application extends="force:slds" controller="LightningAccountController">
    <aura:attribute name="accounts" type="Account[]" />
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />

    <lightning:select label="Choose 1" name="a_opt" aura:id="a_opt" onchange="{!c.change}">
        <aura:iteration items="{!v.accounts}" var="account">
            <option value="{!account.Id}">{!account.Name}</option>
        </aura:iteration>
    </lightning:select>
</aura:application>

// Controller
({
    doInit: function(component, event, helper) {
        // Request from server
        var action = component.get("c.getAccountList");
        action.setCallback(this, function(result){
            var accounts = result.getReturnValue();
            console.log(accounts);
            component.set("v.accounts", accounts);
            // Let DOM state catch up.
            window.setTimeout(
                $A.getCallback( function() {
                    // Now set our preferred value
                    component.find("a_opt").set("v.value", accounts[4].Id);
                }));
        });
        $A.enqueueAction(action);
    }
})

// Apex class
global class LightningAccountController {
    @AuraEnabled global static Account[] getAccountList() {
        return [SELECT Name FROM Account LIMIT 10];
    }
}