Donald is calculating the number of rooms available for sale. This is typically referred to as "out-of-order rooms." However, in the context provided, it seems he is focusing on the number of rooms that can be sold to new guests, which is often referred to as "available rooms" after accounting for those already reserved or occupied.
If we had to select from the options provided, the most fitting one based on the description would be "out-of-order rooms," as he is determining the available rooms that can be sold.