The query performs a LEFT JOIN between the 'sales' and 'favorite_stores' tables on the 'customer_id' field. This means all records from the 'sales' table will be included in the result, and where there is no matching record in 'favorite_stores', the result will contain NULL for the 'store_id'. In this case, the 'sales' table contains records with customer IDs 'a1', 'a3', and 'a4'. The 'favorite_stores' table contains records matching 'a1' and 'a4', but not 'a3'. Therefore, the correct result is to include all rows from the 'sales' table and NULL for the unmatched row, which is represented in the option showing 'a3' with a NULL store_id.