Okay, let's tackle this question. The scenario is a construction worker who fell feet-first, resulting in a fracture of the groove on the undersurface of the sustentaculum tali of the calcaneus. The question is asking which muscle tendon is most likely torn.

First, I need to recall the anatomy of the calcaneus and the structures around it. The sustentaculum tali is a part of the calcaneus, forming a bony shelf. The groove on its undersurface is important for certain tendons passing through or over it.

The muscles that attach to or pass near the sustentaculum tali include the flexor hallucis longus, flexor digitorum longus, tibialis posterior, and maybe others. The sustentaculum tali is also part of the medial longitudinal arch support, along with the spring ligament.

The flexor hallucis longus tendon runs along the medial side of the ankle and is positioned near the sustentaculum tali. If there's a fracture here, the tendon might be affected. The tibialis posterior tendon is more medial and supports the medial arch, but I think it's more associated with the navicular bone. The flexor digitorum longus tendon is adjacent but might be more posterior. The peroneus longus and brevis are lateral, so probably not involved here.

Wait, the sustentaculum tali's groove specifically relates to the flexor hallucis longus tendon. The flexor hallucis longus tendon runs in the groove of the sustentaculum tali. So if that area is fractured, the FHL tendon is at risk. The tibialis posterior tendon is more involved in the talar dome and the navicular. So the correct answer should be the flexor hallucis longus.

**Core Concept**
The sustentaculum tali is a bony projection of the calcaneus that forms part of the medial ankle and supports the medial longitudinal arch. The **flexor hallucis longus (FHL)** tendon runs in a groove on its undersurface, making it vulnerable to injury in fractures of this region.

**Why the Correct Answer is Right**
The **flexor hallucis longus (FHL) tendon** is uniquely positioned in the groove of the sustentaculum tali. A fracture here directly compromises this tendon, which flexes the great toe and everts the foot. The FHL originates from the fibula and interosseous membrane, courses posterior to the medial malleolus, and passes under the sustentaculum tali. Disruption of this groove can tear the tendon due to its fixed course and proximity to the fracture site.

**Why Each Wrong Option is Incorrect**
**Option A:** *Tibialis posterior* tendon: This tendon supports the medial arch but inserts on the navicular and cuneiforms, not the sustentaculum tali.
**Option B:** *Flexor digitorum longus (FDL)* tendon: It runs posterior to the FHL but does not pass through the sustentaculum tali groove.
**Option C:** *Peroneus brevis* tendon: Located laterally, it stabilizes the lateral ankle and is unrelated to the medial sustentaculum tali.
**Option D:** *Ach

✓ Correct Answer: D. Flexor hallucis longus