**Core Concept**
The ankle joint stability is maintained by a combination of static and dynamic factors, including ligaments, muscles, and bones. The static factors include the medial and lateral collateral ligaments, as well as the anterior and posterior talofibular ligaments.

**Why the Correct Answer is Right**
The anterior talofibular ligament (ATFL) is the most commonly injured ligament in ankle sprains. It provides medial stability to the ankle joint by preventing excessive eversion of the foot. The ATFL connects the talus to the fibula and helps maintain the joint's integrity during weight-bearing activities.

**Why Each Wrong Option is Incorrect**
**Option A:** The deltoid ligament, also known as the medial collateral ligament, is a strong ligament that provides medial stability to the ankle joint. It helps resist excessive inversion of the foot and maintain joint integrity.

**Option B:** The posterior talofibular ligament (PTFL) is a ligament that provides posterior stability to the ankle joint. It helps prevent excessive posterior displacement of the talus and maintains joint integrity.

**Option C:** The calcaneofibular ligament (CFL) is a ligament that provides lateral stability to the ankle joint. It helps resist excessive eversion of the foot and maintain joint integrity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the ATFL is the most commonly injured ligament in ankle sprains, and its injury can lead to chronic ankle instability.

**Correct Answer: A. Anterior talofibular ligament.**