What constraint should be applied on a table column so that duplicate values are not allowed in that column

Question

A) What constraint should be applied on a table column so that duplicate values are not allowed in that column, but NULL is allowed.

OR

B) What constraint should be applied on a table column so that NULL is not allowed in that column, but duplicate values are allowed.

Answer 1

A) UNIQUE constraint should be applied on a table column so that duplicate values are not allowed in that column, but NULL is allowed.

OR 

B) NOT NULL constraint should be applied on a table column so that NULL is not allowed in that column, but duplicate values are allowed.