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:

CREATE INDEX tablea_idx_col1_col2_col4_col5 ON tableA (col1,col2,col4,col5);
CREATE INDEX tablea_idx_col4_col5 ON tableA (col4,col5);
CREATE INDEX tablec_idx_resultcode ON tableC (ResultCode);

The optimized query:

SELECT
        a.col1,
        a.col2,
        (SELECT
            COUNT(col3) 
        FROM
            tableA ta 
        INNER JOIN
            tableB b 
                ON tl.TaskId = b.col6 
        INNER JOIN
            tableC c 
                ON b.Id = c.TaskId 
        WHERE
            c.ResultCode = 1 
            AND ta.col4 = a.col4 
            AND ta.col5 = a.col5) AS Executed,
        (SELECT
            COUNT(col3) 
        FROM
            tableA ta 
        INNER JOIN
            tableB b 
                ON tl.TaskId = b.col6 
        INNER JOIN
            tableC c 
                ON b.Id = c.TaskId 
        WHERE
            c.ResultCode = 9 
            AND ta.col4 = a.col4 
            AND ta.col5 = a.col5) AS NotExecuted 
    FROM
        tableA a 
    GROUP BY
        a.col1,
        a.col2,
        a.col4,
        a.col5