**Core Concept**
The external oblique muscle is a key abdominal wall muscle involved in trunk stabilization, lateral flexion, and assisted respiration. It originates from the outer surfaces of the lower eight ribs and inserts into the iliac crest and pubic tubercle, playing a role in rotational and flexion movements of the vertebral column and contributing to abdominal wall integrity.

**Why the Correct Answer is Right**
- **Anterior flexion of vertebral column**: The external oblique, when contracted on one side, causes lateral flexion and rotation, but when both sides contract synergistically, it contributes to anterior flexion of the trunk.
- **Active expiration**: During forced exhalation, the external oblique assists in increasing intra-abdominal pressure, thereby promoting active expiration.
- **Closure of inguinal ring**: The external oblique, along with internal oblique and transversus abdominis, helps close the inguinal ring during abdominal pressure increases, preventing herniation.

**Why Each Wrong Option is Incorrect**
Option A: While the external oblique contributes to trunk flexion, anterior flexion is more accurately attributed to the rectus abdominis and intercostal muscles. The external oblique is not the primary mover for this action.
Option B: Active expiration is primarily driven by the internal intercostals and abdominal muscles; while external oblique contributes, it is not the primary or defining action.
Option C: Closure of the inguinal ring is a function of the external oblique, internal oblique, and transversus abdominis — thus, this action is valid, not incorrect.

**Clinical Pearl / High-Yield Fact**
The external oblique is the most superficial of the abdominal muscles and is a key player in the "abdominal wall" mechanism during coughing, vomiting, and straining. Its contraction helps stabilize the spine and prevents inguinal hernias.

✓ Correct Answer: D. All of the above