Which statement best describes the relationship between an allele and a gene?

a. an allele is a variation of a gene that can be expressed as a phenotype.

b. an allele is the part of a gene that attaches to messenger rna molecules.

c. an allele is the primary protein made by a gene found in a developing embryo.

d. an allele is a segment of a dna molecule that controls replication of a gene.