Step 1: The stomach is the most common site for extranodal lymphoma. Nearly all gastric lymphomas are B-cell lymphomas of mucosa-associated lymphoid tissue (MALT lymphoma), a type of non-Hodgkin lymphoma. So option a is true.
Step 2: About 80% of cases are associated with chronic gastritis and H. pylori infection. Eradicating H. pylori with antibiotics induces durable remission, confirming a direct causal relationship. So option c is true.
Step 3: Diagnosis is established by endoscopic biopsy, so option b is true.
Step 4: Since options a, b and c are all correct, the best answer is option d, All of the above.