Choice "A" is correct. Regardless of the type of inventory costing method, the first step would be to calculate the number of units in the ending inventory and/or cost of goods sold. In this question, there are 20,000 units available for sale and thus 10,000 units (20,000 total purchases - 10,000 ending inventory) in cost of goods sold. The (weighted) average purchase price of the units available for sale is ((10,000 x $3) + (5,000 x $4) + (5,000 x $5) / (10,000 + 5,000 + 5,000)$3.75). Thus the cost of goods sold is 10,000 x $3.75$37,500.
Choice "b" is incorrect. In a (weighted) average pricing inventory costing system, the cost of goods sold would not be at the price of the beginning inventory, or $3 in this question, even though there were 10,000 units sold and 10,000 units in the beginning inventory.
Choice "c" is incorrect. In a (weighted) average pricing inventory costing system, the cost of goods sold would not be costed at the price of the first purchase, or $4 in this question.
Choice "d" is incorrect. In a (weighted) average pricing inventory costing system, the cost of goods sold would not be costed at the selling price of the goods, or $10 in this question.
Note that this question does not use good terminology. The question asks about the "value" of the inventory. It should ask about the "cost" of the inventory. The value of the inventory is whatever the inventory could be sold for in the market.