An allele is said to be dominant if:

The correct option is C It is expressed in both homozygous and heterozygous conditions.

Alleles are the two individual genes in a particular gene pair that occupy the same locus or position on a homologous pair of chromosomes. An allele may be dominant or recessive. If the allele is expressed in both homozygous and heterozygous conditions, then it is a dominant allele.

So, the correct answer is option C.