Alleles are:​

A) ​the cell nucleus
B) ​structures that carry specific proteins
C) ​alternate forms of a single gene
D) ​pieces of DNA

DNA

Deoxyribonucleic acid, the self-replicating material present in nearly all living organisms, serving as the carrier of genetic information.

Alleles

Variants or versions of a gene found at the same place on a chromosome that can lead to differences in the physical or physiological traits expressed by that gene.

Single Gene

Refers to a trait or condition determined by a single gene in an organism's genome.