Step 1: Recall Bleuler's framework. He split schizophrenia symptoms into fundamental and accessory groups, and the fundamental set is the famous quartet of A's.
Step 2: Match each option to the quartet: loosening of association is association disturbance, affect disturbance and autism are listed directly, so three options are fundamental symptoms.
Step 3: $Hallucinations$ falls in the accessory category together with delusions and negativism, so it is the odd one out being asked for.
\[\boxed{Hallucinations}\]