**Core Concept:**
The term 'Triceps surae' refers to a group of three calf muscles located on the posterior aspect of the lower leg. These muscles are essential for maintaining balance, propelling the body during walking or running, and preventing ankle joint from collapsing inward.

**Why the Correct Answer is Right:**
The correct answer, **D.** Gastrocnemius, is a part of the Triceps surae complex. The gastrocnemius muscle is a large, two-headed muscle that attaches from the femur to the calcaneus bone in the heel. When contracted, it flexes the ankle plantar flexes and dorsiflexes the subtalar joint, contributing to the overall function of Triceps surae.

**Why Each Wrong Option is Incorrect:**
A. **Option A (Soleus):** Although also a part of Triceps surae, soleus muscle is smaller and single-headed, differing from the double-headed gastrocnemius. Soleus primarily serves to flex the ankle plantar flexes and inverts the foot.

B. **Option B (Flexor hallucis longus):** This muscle is a separate muscle, not a component of Triceps surae. Flexor hallucis longus primarily flexes the ankle and the fifth metatarsophalangeal joint.

C. **Option C (Flexor digitorum longus):** This muscle is another separate muscle, not part of Triceps surae complex. Flexor digitorum longus primarily flexes the lesser toes, not contributing to the ankle joint movements.

**Clinical Pearl:**
Understanding the anatomy and function of Triceps surae complex is crucial in assessing lower limb conditions such as muscle imbalances, weakness, or pain in the calf region. Correct identification of the individual muscles within Triceps surae is essential for proper diagnosis and treatment planning.