A holiday sale is triggered when a holiday falls within the month and there is excess merchandise. (A) describes a situation where only one of these conditions is met (a holiday falls within the month, but there's no extra merchandise). Since one of the two conditions is absent, we can't conclude from this rule that a holiday sale would be declared.

This claims that if no holiday sale is running, we can conclude it must be August. It's true that in August, the conditions that trigger a holiday sale (a holiday in the month and excess merchandise) aren't met. But does that mean August is the only month where a holiday sale might not run? Not at all. Consider a non-August month where no holiday falls within it. The trigger for a holiday sale wouldn't be activated in that month either. The stimulus gives us no reason to think August is unique in this respect. Many months could potentially lack a holiday sale.

This tries to create a link between manager's sales and the absence of excess merchandise, but these are governed by completely separate rules. A manager's sale is triggered by a manager wanting to clear out a line of merchandise. That has nothing to do with the level of merchandise in the warehouse. A manager could want to clear out a line of merchandise in a month when there also happens to be excess merchandise in the warehouse. Nothing in the rules prevents both conditions from being true at the same time.

This follows directly from the first rule: at all times there is either a manager's sale or a holiday sale or both. If at least one type of sale must be running every month, then whenever a manager's sale is not running, a holiday sale must be running. Otherwise, there would be a month with no sale at all, which violates the rule. (D) is simply restating this rule in conditional form.

This claims that anytime there's no excess merchandise in the warehouse, it must be August. But although we know that August never has excess merchandise, that doesn't mean August is the only month that can lack excess merchandise. Other months might also happen to have no excess merchandise in the warehouse. The stimulus tells us a specific fact about August; it doesn't tell us that this fact is exclusive to August.