OriginalGriff's answer is correct.
However, you should also simplify your query like this:
SELECT
b.id AS ID,
b.firstname As FIRSTNAME,
b.lastname AS LASTNAME,
b.middlename AS MIDDLENAME,
w.class_name AS CLASS,
p.passport_img AS PASSPORT
FROM
base_user_details b Left JOIN passport p ON
b.id = p.id
LEFT JOIN ward_class w ON
b.id=w.id
WHERE
(@SelectOption == 'class' AND b.class_name = @filter)
OR (@SelectOption == 'Firstname' AND b.firstname = @filter)
OR (@SelectOption == 'Lastname' AND b.lastname = @filter)