This question tests the understanding of the past perfect tense, which is used to describe actions that were completed before another past action. The correct answer is A because 'had been eating' indicates an action that was in progress before another past action (arrived). Option B is in the present perfect continuous tense, which is incorrect. Option C is in the simple past tense and does not indicate an action completed before another past action. Option D is in the present perfect continuous tense, which is also incorrect.