**Core Concept**
Heller's cardiomyotomy is a surgical procedure that involves cutting the muscular wall of the lower esophageal sphincter (LES) to alleviate symptoms of achalasia. This condition is characterized by the failure of the LES to relax, leading to difficulty swallowing and regurgitation of food.

**Why the Correct Answer is Right**
The procedure is specifically indicated for achalasia, a motility disorder of the esophagus. During the operation, the surgeon makes a longitudinal incision in the LES to relieve the obstruction and allow for normal passage of food. This is achieved by disrupting the abnormal muscle fibers that prevent the sphincter from relaxing.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Heller's cardiomyotomy is not performed for gastroesophageal reflux disease (GERD), which is treated with medications or antireflux surgery.
**Option B:** Incorrect because the procedure is not used to treat esophageal cancer, which may require more extensive surgical resection.
**Option C:** Incorrect because achalasia is a distinct clinical entity from diffuse esophageal spasm, which may be treated with medications or other interventions.

**Clinical Pearl / High-Yield Fact**
Achalasia is often associated with a "bird's beak" appearance on barium swallow imaging, where the distal esophagus is narrowed due to the abnormal LES.

**Correct Answer: D. Achalasia.**