Choice "D" is correct.  because joint products that are not salable at the split-off point require additional processing to make them marketable. To allocate joint costs to this type of product, a hypothetical sales value, representing the relative sales value at split-off, is calculated. This is done by subtracting the after-split-off processing costs from the price at point of sale.
Choice "c" is incorrect. Sales price at point of sale reduced by cost to complete is the additional contribution to income generated by completing the product. It is not equal to total costs.
Choice "b" is incorrect. Sales price at point of sale reduced by cost to complete is the additional contribution to income generated by completing the product. It is not equal to joint costs. (If it were, this would be a zero profit situation.)
Choice "a" is incorrect. Selling price less a normal profit margin is generally a cost figure. It is not equal to sales price less the cost to complete, which is the additional contribution to income generated by completing the product. (If it were, this would be a zero profit situation.)