Problem is your query

SELECT @A = a,
       @Ans = b * c
FROM   tbln
WHERE  d = '2018-A'

only initialise your variables @A and @Ans with one row infos. And only that one is updated afterwards To update all, just use:

UPDATE tbln
SET    e = b*c
WHERE  d = '2018-A'

In the first statement you are setting @A = A . Given that column A is the primary key of your table it is uniquely identifying each row. When the first SELECT statement is done executing @A = 1004 .

Then your UPDATE statement executes and is filtering for A = @A which is A = 1004.

If you want to update column E for all rows in the sample then you need to remove your WHERE clause.

SELECT @A = a,

Will only have one value at the end and so you will update only one row

What you need to do is followinf to update all rows with 2018-A

CREATE TABLE tbln
    ([A] int, [B] int, [C] int, [D] varchar(6), [E] int)
;
    
INSERT INTO tbln
    ([A], [B], [C], [D], [E])
VALUES
    (1000, 5, 2, '2018-A', NULL),
    (1001, 15, 63, '2018-A', NULL),
    (1002, 25, 124, '2018-A', NULL),
    (1003, 35, 185, '2018-A', NULL),
    (1004, 45, 246, '2018-A', NULL),
    (1005, 55, 307, '2018-A', NULL),
    (1000, 65, 368, '2018-B', NULL),
    (1001, 75, 429, '2018-B', NULL),
    (1000, 85, 490, '2018-C', NULL),
    (1001, 95, 551, '2018-C', NULL),
    (1002, 105, 612, '2018-C', NULL),
    (1003, 115, 673, '2018-C', NULL),
    (1004, 125, 734, '2018-C', NULL)
;

13 rows affected

UPDATE tbln
SET    e = b*c
WHERE  d = '2018-A'

6 rows affected

SELECT * FROM tbln

fiddle