Use the Following Information to Answer the Questions Below

Use the following information to answer the questions below.
Skin colour in a certain species of fish is inherited via a single gene with four different alleles.
-One fish of this type has alleles 1 and 3 (S₁S₃) and its mate has alleles 2 and 4 (S₂S₄) . If each allele confers a unit of colour darkness such that S₁ has one unit, S₂ has two units, and so on, then what proportion of their offspring would be expected to have five units of colour?

A) 1/4
B) 1/5
C) 1/8
D) 1/2
E) 0