The correct answer is $\boxed{\text{B. }2,000}$.
The order quantity is the amount of an item that is ordered each time an order is placed. It is calculated by taking the gross requirements, subtracting the inventory on hand, and adding the safety stock. In this case, the gross requirements are 2,000, the inventory on hand is 3,000, and the safety stock is 500. Therefore, the order quantity is $2,000 – 3,000 + 500 = 2,000$.
Option A is incorrect because it is the inventory on hand, not the order quantity.
Option C is incorrect because it is the sum of the gross requirements, inventory on hand, and safety stock.
Option D is incorrect because it is zero. The order quantity cannot be zero, because that would mean that no items are ordered.
Option E is incorrect because it is not one of the possible answers.