Identify the correctly constructed ALTER TABLE statement to add a UNIQUE constraint to the column user$\_$number with the constraint name user$\_$number$\_$unique on the table called 'user'.

a$.)$

ALTER TABLE user ADD CONSTRAINT user$\_$number UNIQUE $($user$\_$number$\_$unique$)$;

b$.)$

ALTER TABLE user ADD CONSTRAINT user$\_$number UNIQUE $($user$\_$number$)$;

c$.)$

ALTER TABLE user ADD UNIQUE user$\_$number CONSTRAINT $($user$\_$number$\_$unique$)$;

d$.)$

ALTER TABLE user ADD CONSTRAINT user$\_$number$\_$unique UNIQUE $($user$\_$number$)$;