To display unique promotion costs in each promotion category, the correct queries that can be used are:
* C. SELECT DISTINCT promo_category ||' has '|| promo_cost AS COSTS FROM promotions ORDER BY 1;This query concatenates the promo_category with the literal ' has ' and promo_cost, giving a unique string for each combination of promo_category and promo_cost, which is what we are interested in when we want to list unique costs per category.
* D. SELECT DISTINCT promo_category, promo_cost FROM promotions ORDER BY 1;This query selects distinct combinations of promo_category and promo_cost, which is exactly what's required to display unique promotion costs in each category.
Options A, B, and E are incorrect:
* A is incorrect because it does not use the DISTINCT keyword to ensure uniqueness.
* B has incorrect syntax; the DISTINCT keyword should appear directly after SELECT and applies to all columns, not just one.
* E is syntactically incorrect and would not execute because the DISTINCT keyword is not used correctly.
It should not appear twice in the SELECT clause.