Final answer:
The uterus is not composed of skeletal muscle but of smooth muscle, distinguishing it from the quadriceps, triceps, and gastrocnemius, which are skeletal muscles.
Step-by-step explanation:
The muscle in the list that is not composed of skeletal muscle is the uterus. While the quadriceps, triceps, and gastrocnemius are all skeletal muscles, which are under voluntary control and support movement of the skeleton, the uterus is composed of smooth muscle. Smooth muscle is involuntary and is found in various internal organs. In a multiple-choice question like this, it's important to identify the one that differs in function and composition from the others.