It's possible the retailer doesn't sell a thing, so the information about replenishing inventory can't logically affect the answer (if it did, there would be different answers in different situations, and thus more than one right answer to the question, which can't happen on the GMAT). If the monthly average is y/2 items, the monthly cost is (0.5)(y/2) = y/4 dollars, and the yearly cost is 12 times that, or $3y.
There seem to be issues with the setup though. It's not clear what is even being averaged, for one thing, or what it means to "maintain y/2 in inventory", nor how the inventory cost is assessed - there's a confusion between units, between days and months. If a unit is only in inventory for part of a month, does the retailer pay for it? That's not at all clear - I could easily take "The cost of keeping an item in inventory is $0.5 per month" to mean that inventory only costs the retailer money when it is "kept" in inventory for a month. And the question says the average is "estimated", but then goes on to ask for an exact answer. So I think there are several issues with the question design.