**Core Concept**
The anterior talofibular ligament (ATFL) is a critical ligament that provides stability to the ankle joint. It is one of the three ligaments that form the lateral collateral ligament complex of the ankle, which also includes the calcaneofibular ligament and the posterior talofibular ligament.

**Why the Correct Answer is Right**
The ATFL is the most commonly injured ligament in ankle sprains, particularly in inversion injuries. This is due to its anatomical location, which makes it prone to stretching and tearing when the foot is forced inward. The ATFL is a thin, fibrous ligament that connects the talus and fibula bones, and it plays a crucial role in preventing excessive movement of the ankle joint.

**Why Each Wrong Option is Incorrect**
* **Option A:** The posterior talofibular ligament (PTFL) is also a ligament of the lateral collateral ligament complex, but it is less commonly injured than the ATFL.
* **Option B:** The calcaneofibular ligament (CFL) is another ligament of the lateral collateral ligament complex, but it is not the most commonly injured ligament in ankle sprains.
* **Option C:** The deltoid ligament is a ligament of the medial collateral ligament complex, which is located on the opposite side of the ankle joint from the lateral collateral ligament complex.

**Clinical Pearl / High-Yield Fact**
The ATFL is often injured in combination with other ligaments, and it is essential to assess for injuries to the other ligaments in the lateral collateral ligament complex.

**Correct Answer: A. Anterior talofibular ligament.