Final answer:
The biceps femoris is not a knee extensor; it is a knee flexor, and part of the hamstring group. The other options listed are all knee extensor muscles as they form the quadriceps femoris group.
Step-by-step explanation:
The muscle among the options that is not a knee extensor is the biceps femoris. The biceps femoris is one of the muscles comprising the hamstring group, which is primarily responsible for knee flexion, not extension. The other muscles listed, including the vastus intermedius, vastus medialis, rectus femoris, and vastus lateralis, are all part of the quadriceps femoris group and are indeed knee extensors.