Vishnu thought the last problem on the test was a real __________. It was much harder and more complex than any of the previous problems.

'Doozy' is a term used to describe something unique. 
'Whooper' is a term used to describe a large swan found in Europe, Asia, and Greenland. The term is also used to indicate a person who makes a loud noise (Like the swan). 
'Carp' means to find fault with something.
'Snafu' is a term used to describe a muddled situation.

We can infer from the sentence that Vishnu thought the problem to be one of its kind. Therefore, 'doozy' is the correct term to be used and hence, option A is the right answer.