Which of the following are muscles?

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The selection of buccinator, gluteus maximus, and gastrocnemius as muscles is correct because each of these terms refers specifically to types of muscle tissue in the human body. The buccinator is a facial muscle that helps in actions such as blowing and chewing, the gluteus maximus is a major extensor of the hip that plays a critical role in maintaining an upright posture and allowing for movements like climbing stairs, and the gastrocnemius is a prominent muscle in the calf that is vital for actions like walking, running, and jumping. All three are integral to various functions related to movement and stability.

The other options do not contain muscles; rather, they consist of different types of tissues or organs. The liver, kidney, and pancreas are all vital organs involved in various metabolic processes, while neurons, synapses, and neurotransmitters pertain to the nervous system's structure and function. The last group includes structures related to the skeletal system—the pelvis, femur, and tibia—which are bones, not muscles. Thus, the first option distinctly identifies components that are categorized as muscles.