A. Drop the UDF and rewrite the report query as follows:
SELECT c.CustomerName
FROM Sales.Customer c
WHERE NOT EXISTS (
SELECT s.OrderDate
FROM Sales.SalesOrder s
WHERE s.OrderDate > DATEADD(DAY, -90, GETDATE())
AND s.customerID = c.CustomerID)

B. Rewrite the report query as follows:
SELECT c.CustomerName
FROM Sales.Customer c
WHERE (SELECT OrderDate FROM Sales.ufnGeTLastOrderDate(c.CustomerID)) < DATEADD(DAY, -90, GETDATE()) Rewrite the UDF as follows:
CREATE FUNCTION Sales.ufnGeTLastOrderDate(@CustomerID int)
RETURNS @OrderTable TABLE (OrderDate datetime)
AS
BEGIN
INSET @OrderTable
SELECT MAX(s.OrderDate) As OrderDate
FROM Sales.SalesOrder s
WHERE s.CustomerID = @CustomerID
RETURN
END

C. Rewrite the report query as follows:
SELECT c.CustomerName
FROM Sales.Customer c
WHERE NOT EXISTS (SELECT OrderDate FROM Sales.ufnGetRecentOrders
(c.CustomerID, 90))
Rewrite the UDF as follows:
CREATE FUNCTION Sales.ufnGetRecentOrders(@CustomerID int, @MaxAge datetime) RETURNS TABLE AS RETURN ( SELECT OrderDate FROM Sales.SalesOrder s WHERE s.CustomerID = @CustomerID AND s.OrderDate > DATEADD(DAY, -@MaxAge, GETDATE()) )

D. Drop the UDF and rewrite the report query as follows:
SELECT c.CustomerName
FROM Sales.Customer c
INNER JOIN Sales.SalesOrder s ON c.CustomerID = s.CustomerID
GROUP BY c.CustomerID
HAVING MAX (s.OrderDate) < DATEADD(DAY, -90, GETDATE())