Answer:
depletion expense is recognized on the 1,200,000 tons extracted and sold.
Explanation:
Since the mine extracted 1,500,000 tons but was only able to sell 1,200,000. The 300,000 tons that were not sold should not be used to recognize depletion expense for the current period. Those tons remain "in stock" and can be sold on a later date, on which they should be used to recognized depletion expense.
Therefore, depletion expense is recognized on the 1,200,000 tons extracted and sold.