public class ContactsController {
    @AuraEnabled
    public static List<String> getDepartment () {
        Set<String> uniqDepartment = new Set<String>();
        List<Contact> results=[Select Id, Department from Contact];
        for(Contact c : results){
            if(c.Department!=null){
                uniqDepartment.add(c.Department); 
            }
        }
        List<String> finalResult =  new List<String>();
        finalResult.addAll(uniqDepartment); 
        finalResult.sort();
        return  finalResult;
    }
    
    @AuraEnabled
    public static List<Contact> getContactsByDept (String deptName) {
        List<Contact> results=[Select Id,FirstName,LastName,Email,Department from Contact where Department=:deptName];
        return  results;
    }
    
    
    
}

<aura:component controller="ContactsController">
    <aura:attribute name="statusOptions" type="List" default="[]"/>
    <aura:attribute name="contacts" type="Contact[]" default="[]"/>
    
    <aura:handler name="init" value="{! this }" action="{! c.loadOptions }"/>
    <lightning:combobox aura:id="selectItem" name="status" label="Status"
                        placeholder="Choose Status"
                        value="new"
                        required="true"
                        dropdownAlignment="right"
                        variant="standard"
                        messageWhenValueMissing="You Must Select the Value"
                        onchange="{!c.handleOptionSelected}"
                        options="{!v.statusOptions}"/>
    
    <table class="slds-table slds-table--bordered">
        <thead>
            <th>
                LastName 
            </th>
            
            <th>
                FirstName
            </th>
            <th>
                Email
            </th>
        </thead>
        <tbody>
            <aura:iteration items="{!v.contacts}" var="row">
                <tr>
                    <td>
                        {!row.LastName}
                    </td>
                    <td>
                        {!row.FirstName}
                    </td>
                    <td>
                        {!row.Email}
                    </td>
                </tr>
            </aura:iteration>
        </tbody>
    </table>
    
    
</aura:component>

({
    loadOptions: function (cmp, event, helper) {
        var options = [
        ];
        var action = cmp.get("c.getDepartment");
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                var arr = response.getReturnValue() ;
                arr.forEach(function(element) {
                    options.push({ value: element, label: element });
                });
                cmp.set("v.statusOptions", options);
                
            }
            
            
        });
        $A.enqueueAction(action);
        
    },
    handleOptionSelected: function (cmp, event) {
        var action = cmp.get("c.getContactsByDept");
        action.setParams({deptName:event.getParam("value")});
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                cmp.set("v.contacts" ,  response.getReturnValue() ) ; 
            }
        });
        $A.enqueueAction(action); 
        
    }
})