**Core Concept**
The superficial arch of the foot is a complex structure made of bones, ligaments, and tendons that provides support and stability to the foot. The tendinous sling is a critical component of this arch, playing a key role in maintaining its shape and function.

**Why the Correct Answer is Right**
The tendinous sling in the superficial arch of the foot is formed by the peroneus longus tendon, which passes under the cuboid bone and wraps around the medial cuneiform bone. This unique arrangement creates a sling-like structure that helps to support the arch and maintain its shape. The peroneus longus tendon is a key player in this process, as it provides the necessary tension to keep the arch stable.

**Why Each Wrong Option is Incorrect**
**Option A:** The peroneus brevis tendon is not involved in forming the tendinous sling of the superficial arch of the foot. It attaches to the base of the fifth metatarsal bone and does not contribute to the arch's stability.
**Option B:** The flexor digitorum longus tendon is not part of the superficial arch of the foot. It is a muscle tendon that inserts into the distal phalanges of the toes and is not involved in the formation of the arch.
**Option C:** The Achilles tendon is not related to the superficial arch of the foot. It is a tendon that connects the gastrocnemius and soleus muscles to the calcaneus bone and is involved in ankle movement, not foot arch stability.

**Clinical Pearl / High-Yield Fact**
The peroneus longus tendon is a key structure to remember when assessing foot mechanics and stability. Its unique arrangement in the superficial arch of the foot makes it a critical component of foot function, and damage to this tendon can lead to flatfoot deformity and other related issues.

**Correct Answer: C. Peroneus longus tendon**