//HTML
<aura:component implements="flexipage:availableForRecordHome,
                            force:hasRecordId" access="global" 
                controller="fetchContact">
    <aura:handler name="init" value="{!this}" action="{!c.doInIt}"/>
    <aura:attribute name="selectedContact" type="String" />
    <aura:attribute name="conOption" type="contact[]" />
    <div>
                    
        <lightning:select name="con" 
                          class="slds-combobox_container slds-size_small"  
                          label="Contact" 
                          aura:id="con" value="{!v.selectedContact}" onchange="{!c.changeAction}" >
                       <option value="">choose one...</option>
                 <aura:iteration items="{!v.conOption}" var="opt">
                     {!opt.LastName}
                     <!--<option text="{!opt.LastName}" value="{!opt.Id}" selected="{!opt.selected}" /> -->
                      <option value="{!opt.Id}">{!opt.LastName}</option>
               </aura:iteration>
               </lightning:select>
                      </div>  
    selectedContact : {!v.selectedContact}

</aura:component>

//Controller
({
	doInIt : function(component, event, helper) {
        var action = component.get("c.fetchRecord");
        
        action.setCallback(this, function(response) {
            var state = response.getState();
            alert('state : ' + state);
            if (state === "SUCCESS") {
                var fetchRecords = response.getReturnValue();
                console.log("fetchRecords From server: " + JSON.stringify(fetchRecords));
            component.set('v.conOption',fetchRecords);      
            }
            else if (state === "INCOMPLETE") {
                // do something
            }
            else if (state === "ERROR") {
                var errors = response.getError();
                if (errors) {
                    if (errors[0] && errors[0].message) {
                        console.log("Error message: " + 
                                 errors[0].message);
                    }
                } else {
                    console.log("Unknown error");
                }
            }
        });
 
        $A.enqueueAction(action);
		
	},
    changeAction : function(component, event, helper) {
        var conId = event.getSource().get('v.value');
        alert(conId);
        component.set('v.selectedContact',conId);
    }
})

//Apex
public with sharing class fetchContact {
     @AuraEnabled 
    public static list<Contact> fetchRecord(){
        return [SELECT Id,FirstName,LastName From Contact limit 10];
        
    }
}