Please Use the Following Information to Answer the Question(s) Below

Please use the following information to answer the question(s) below.
Skin color 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 color 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 color?

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