The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.

Optimal indexes for this query:

ALTER TABLE `Cities` ADD INDEX `cities_idx_cityid` (`CityID`);
ALTER TABLE `Countries` ADD INDEX `countries_idx_countryid` (`CountryID`);
ALTER TABLE `Listings` ADD INDEX `listings_idx_isactive_isapproved` (`IsActive`,`IsApproved`);
ALTER TABLE `provinces` ADD INDEX `provinces_idx_provinceid` (`ProvinceID`);

The optimized query:

SELECT
        DISTINCT p.ProvinceID,
        p.Abbv AS RegionCode,
        p.name AS RegionName,
        cn.Code AS CountryCode,
        cn.Name AS CountryName 
    FROM
        dbo.provinces AS p 
    INNER JOIN
        dbo.Countries AS cn 
            ON p.CountryID = cn.CountryID 
    INNER JOIN
        dbo.Cities c 
            ON c.ProvinceID = p.ProvinceID 
    INNER JOIN
        dbo.Listings AS l 
            ON l.CityID = c.CityID 
    WHERE
        l.IsActive = 1 
        AND l.IsApproved = 1