What is the primary action of the iliacus muscle?

The iliacus muscle is primarily responsible for flexing the thigh at the hip joint. This action is essential for various movements, such as walking, running, and climbing, where bringing the thigh closer to the abdomen is required. Additionally, the iliacus assists in medial rotation of the thigh, contributing to better balance and stability during these movements.

While the iliacus is mainly known for its role in flexing the thigh, its anatomical position allows it to also participate in medial rotation when the thigh is flexed. The relationship between flexion and medial rotation is significant in many functional activities, underscoring why option B is the most accurate representation of the iliacus muscle's primary actions.