The correct option is A Gene
Allele is an alternative form of a gene. That is, a gene can express if and only if its pair is available - and that pair is otherwise called as allele.
These are located on the same chromosome and thus transferred to further generation through gametes.
It is represented in the form of letters. For example, when the is phenotype is tall, genotypes possible are TT and Tt. Here Tallness is the dominant trait and hence the allele T is called as dominant allele.