The correct answer is: C. portion of production of accounting period.
The sales value at split-off method is a method of accounting for joint products. It is used when the joint products are sold at the split-off point, and the costs of production are allocated to the joint products based on their relative sales value at the split-off point.
Under the sales value at split-off method, the value of sales considers the sales value of the portion of production that is sold at the split-off point. The costs of production that are allocated to the portion of production that is sold at the split-off point are then deducted from the sales value to determine the gross profit from the sale of the portion of production that is sold at the split-off point.
The other options are incorrect because they do not consider the sales value of the portion of production that is sold at the split-off point. Option A considers the entire direct material of the accounting period, but this includes the direct material that is used in the production of the portion of production that is not sold at the split-off point. Option B considers the entire production of the accounting period, but this includes the production of the portion of production that is not sold at the split-off point. Option D considers the entire indirect material of the accounting period, but this includes the indirect material that is used in the production of the portion of production that is not sold at the split-off point.