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

a$.)$

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

b$.)$

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

c$.)$

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

d$.)$

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