SQL> CREATE TABLE b1
  2  (l1 NUMBER,
  3   l2 NUMBER);

Table created.
SQL> CREATE TABLE t1
  2  (c1 NUMBER);

Table created.

SQL> INSERT INTO t1
  2  VALUES(1);

1 row created.

SQL> INSERT INTO b1
  2  VALUES(1,2);

1 row created.

SQL> COMMIT;

Commit complete.

Now according to your form,
Post-Query Trigger on Block Level:

DECLARE
	var1 NUMBER;
BEGIN
	SELECT c1
	INTO var1
	FROM t1;
	IF var1 = 1 THEN
		Set_Item_Instance_Property( 'B1.l1', CURRENT_RECORD, VISUAL_ATTRIBUTE,'VA1'); 
	ELSIF var1 = 2 THEN
		Set_Item_Instance_Property( 'B1.l2', CURRENT_RECORD, VISUAL_ATTRIBUTE,'VA2'); 
	END IF;
END;

You can see the result at run-time.

Change the value of c1 to 2 in t1 and the result will be reflected in the forms Run-Time.