Reporting that one of its many problems had been the recent

First difference between the answer choices: "its" vs. "their". "Retailer" is singular (it's referred to as "it" later in the sentence). So all options with "their" (plural) can be eliminated. Watch out for switches between singular and plural in answer choices — it's an easy clue.

Second difference: tense ("had been" vs. "has been" vs. "is"). "Had been" is past perfect (happened before an event in the past), "has been" is present perfect (happened before the present), and "is" is present. The sentence describes how the retailer "said" (past tense) it would start a liquidation sale in response to the extended sales slump. This means that the sales slump happened before the retailed said it would start the liquidation sale — so the slump occurred before an event in the past ("said"). This makes past perfect the correct tense, so we pick A. When you're unsure of tense, making a timeline of the events in the sentence can help clarify when different events occurred relative to other events (especially when dealing with perfect or continuous tenses).