List price = 1.40 × cost. Selling price = 0.90 × list = 0.90 × 1.40 × cost = 1.26 × cost. Profit = selling price − cost = 1.26C − C = 0.26C = $78, so cost = $78 / 0.26 = $300. Check: list = $420, sale = $378, profit = $78.
Choice B, $260, comes from treating a 40% markup followed by a 10% discount as netting to 30% profit. That does not hold, because the discount is taken on the larger marked-up price, not on the original cost. The true margin is 1.4 × 0.9 − 1 = 0.26, not 0.40 − 0.10 = 0.30. Choice A, $195, drops the discount entirely. Choice D, $312, comes from mis-multiplying 1.4 × 0.9 as 1.25 instead of 1.26. Choice E, $780, mistakes the discount rate for the profit margin.