I have to disagree with the correct answer(s). The question asks for 4 correct answers yet 5 are presented. This causes a bit of a guessing game.
I didn't read the last one ; when marked 4th one , I should read complete question and answers ;-)..
yes the last one is correct, Unique key does allow one null.
"The Unique key allows NULL values as a value"
is an answer too..
Wondering what I would do next , when I am done with this one