The study of genes is called genetics.
The correct answer is A.
In the reproduction process, there are two parents involved in the process which leads to the mixing of two different types of a chromosome which have a different sequence of genes.
The mixing of the different genes leads to the formation of new characters by mixing the new genes and forming a new sequence.
The variation in the dogs is because of the variation in the gene sequence.
The genes are present on the chromosome and the chromosome is present inside the nucleus.
Hence, the correct answer is A that is a gene, chromosome, and nucleus.
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