If your friend says they know that someone in your class has a crush on you, but you say you don't believe them, according to signal detection theory, you have committed a

Miss  - According to signal detection theory, there can be four possible outcomes to a stimulus:
a. The information is true and you accept it  (Hit)
b. The information is true and you don't accept it (Miss)
c. The information is false but you accept it as true (false alarm)
d. The information is false and you accept it as false (correct rejection)
In this case, the friend gives correct information, but you don't accept it, so it is miss.