The joint cost allocation method that ensures that the gross margin for each product is identical is the:

A) relative sales value method.
B) net realisable value method.
C) joint cost allocation method.
D) constant gross margin method.

The constant gross margin method allocates joint costs based on the gross margin percentage of each product. This ensures that the gross margin for each product is identical, making it a fair method of allocation. The other methods may not necessarily result in the same gross margin for each product.