What is the category of the product that has the highest number of discontinued products?
discontinued products refers to Discontinued = 1; highest number of discontinued products refers to MAX(Discontinued = 1)
SELECT T2.CategoryName FROM Products AS T1 INNER JOIN Categories AS T2 ON T1.CategoryID = T2.CategoryID WHERE T1.Discontinued = 1 GROUP BY T2.CategoryName ORDER BY COUNT(T1.ProductID) DESC LIMIT 1