The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 20): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `SaleAmount` is indexed, the index won’t be used as it’s wrapped with the function `isnull`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Avoid Calling Functions With Indexed Columns (query line: 21): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `CityId` is indexed, the index won’t be used as it’s wrapped with the function `isnull`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
3. Avoid Calling Functions With Indexed Columns (query line: 22): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `TopLevelCategoryId` is indexed, the index won’t be used as it’s wrapped with the function `isnull`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
4. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
5. 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 `tblBrandDetails` ADD INDEX `tblbranddetails_idx_categoryid_brandid_cityid` (`CategoryId`,`BrandId`,`CityId`);
ALTER TABLE `tblBrands` ADD INDEX `tblbrands_idx_brandid` (`BrandId`);
ALTER TABLE `tblCategory` ADD INDEX `tblcategory_idx_categoryid` (`CategoryId`);
ALTER TABLE `tblCity` ADD INDEX `tblcity_idx_cityid` (`CityId`);

The optimized query:

SELECT
        * 
    FROM
        tblBrandDetailUsers tbdu 
    INNER JOIN
        tblBrands tbs 
            ON tbs.BrandId = tbdu.BrandId 
    LEFT JOIN
        tblBrandDetails tbd 
            ON tbd.CategoryId = tbdu.CategoryId 
            AND tbd.BrandId = tbdu.BrandId 
            AND tbd.CityId = tbdu.CityId 
    INNER JOIN
        tblCategory tc 
            ON tbdu.CategoryId = tc.CategoryId 
    INNER JOIN
        tblCity tcc 
            ON tcc.CityId = tbdu.CityId 
    WHERE
        isnull(tbdu.SaleAmount, -1) <> isnull(tbd.SaleAmount, -1) 
        AND isnull(tbdu.CityId, 0) = 3 
        AND isnull(tbdu.TopLevelCategoryId, 0) = 2