Which of the following should have a hyphen?
a. nineteen
b. american president
c. self evident
d. written quickly

The answer to this is (C), self-evident. The reason why this is the answer is that this is the only answer given that is a compound word, and compound words require hyphens. Answer A is a number and is its own word. Answer B are two separate words that are independent and do not need to be joined. Answer D is a combination of adjective and verb and does on need a hyphen.