The correct answer is: $2,720 lower

If marginal costing is used to value inventory instead of absorption costing, the difference in profits will be equal to the change in inventory volume multiplied by the fixed production overhead absorption rate = 80 units x $34 = $2,720

Since closing inventory are higher than opening inventories, the marginal costing profit will be lower that the absorption costing profit (so $2,720 higher is incorrect). This is because the marginal costing profit does not 'benefit' from the increase in the amount of fixed production overhead taken to inventory (rather than to the income statement).

If you selected $3,400 lower or $3,400 higher you based the difference on 100 units of opening inventory.