An allele is described as ‘dominant’ if it expresses itself when it occurs in

1. Allelomorph is another name for an allele. Alleles are found in pairs and regulate the same traits, such as blood type. 
2. Different alleles have dominant and recessive traits. The degree of resemblance between alleles in an organism is known as zygosity. 
3. A person who is homozygous has inherited the same alleles (DNA sequences) for a given gene from both of their parents. 
4. Here, the alleles are the same. Homozygous for either TT or TT. There are differences between the alleles. Heterozygotes are Tt or tT. 
5. When an allele is present, the trait of the dominant allele always manifests in the organism. A dominant allele hides the existence of a recessive one. 
6. A dominating can therefore be either TT or Tt. So a dominant allele can exist in both homozygous and heterozygous situations.

Hence, option C is the correct answer.