Which muscle is located in the lower leg?

The gastrocnemius is the correct choice as it is one of the primary muscles located in the lower leg. This muscle is part of the calf and plays an essential role in activities such as walking, running, and jumping by facilitating ankle plantarflexion, which is the movement of pointing the toes downward. Additionally, the gastrocnemius muscle crosses both the knee and ankle joints, contributing not only to leg movement but also to the stabilization of the knee.

In contrast, the biceps brachii is located in the upper arm and is responsible for elbow flexion and forearm supination. The rectus abdominus is situated in the abdominal area and functions primarily in flexing the spine. The erector spinae is a group of muscles running along the spine and is crucial for maintaining posture and extending the back. Hence, all of these muscles are unrelated to the region specifically referred to as the lower leg.