Final answer:
The not compound exercise among the options is the seated leg extension as it is an isolation exercise that primarily targets the quadriceps.
Step-by-step explanation:
The exercise that is not considered a compound exercise is C. Seated leg extension. Compound exercises are those that involve multiple joints and muscle groups at the same time. The overhead press, barbell squat, and bench press are all examples of compound exercises as they engage several muscle groups throughout the movement.
In contrast, the seated leg extension is an isolation exercise; it primarily targets the quadriceps in the legs and involves movement around a single joint, the knee. This focus on a single muscle group without significantly involving other muscles is what sets it apart from the compound movements listed.