D. Peptic stricture: This is the most likely diagnosis given the patient's history of retrosternal discomfort and the use of H2 blockers for a year, suggesting chronic gastroesophageal reflux disease (GERD). Peptic stricture is a known complication of long-standing GERD, which can cause progressive dysphagia due to narrowing of the esophagus.

C. Pharyngeal pouch: Although less common, a pharyngeal pouch (Zenker's diverticulum) could cause progressive dysphagia. It is more likely to cause intermittent symptoms and might not directly relate to the patient's history of retrosternal discomfort.
B. Plummer-Vinson syndrome: This syndrome is characterized by dysphagia, iron-deficiency anemia, and esophageal webs. While the patient's hemoglobin level is within the normal range, this does not entirely rule out the syndrome, especially if the patient has been experiencing recent onset of symptoms or if the anemia is mild.

E. Esophageal Cancer: Although the patient is relatively young and has not noticed any weight loss, esophageal cancer must be considered due to its potential severity and the fact that dysphagia is a key symptom. Early diagnosis is crucial for treatment outcomes.
A. Foreign body: While less likely in an adult without a history of ingestion or other suggestive symptoms, a foreign body could cause sudden or progressive dysphagia and should be considered, especially if the onset of symptoms was rapid.

Other rare causes of dysphagia, such as esophageal rings, eosinophilic esophagitis, or motility disorders like achalasia, could be considered but are less likely given the patient's presentation and history. These would typically require further investigation, including endoscopy and manometry, for diagnosis.