**Core Concept**
The question is testing the student's knowledge of muscles with dual innervation and function, specifically those that act on both the knee and ankle joints. This requires an understanding of the anatomy and biomechanics of the lower limb.

**Why the Correct Answer is Right**
The gastrocnemius muscle is a flexor of the foot at the ankle joint and a flexor and extensor of the knee joint. It has a dual innervation from both the tibial nerve (for the distal portion) and the common peroneal nerve (for the proximal portion). This unique arrangement allows the gastrocnemius to contribute to both knee and ankle movements.

**Why Each Wrong Option is Incorrect**
* **Option A:** Gastrocnemius is the correct answer, so we'll explain the other options.
* **Option B:** Soleus primarily acts on the ankle joint as a flexor and is not involved in knee movement.
* **Option C:** Plantaris is a small muscle that assists in flexion of the ankle and knee, but it is not a primary muscle acting on both joints.
* **Option D:** Tibialis posterior is a muscle that primarily acts on the ankle joint, with some contribution to foot inversion and support, but it is not a primary flexor of the knee.
* **Option E:** Flexor hallucis longus primarily acts on the great toe, contributing to its flexion, and is not a primary muscle acting on either the knee or ankle.

**Clinical Pearl / High-Yield Fact**
When evaluating lower limb injuries or conditions, it's essential to remember that the gastrocnemius muscle has a dual innervation, which can sometimes lead to confusion in diagnosing nerve injuries or entrapments.

**Correct Answer:** A. Gastrocnemius