The egg cells of a dog have 39 chromosomes, which means that this is the haploid number of chromosomes (n). In dogs, the diploid number of chromosomes (2n), which is the number of chromosomes found in body cells, would be double that of the haploid number. So, for a dog, the diploid number would be:
2n = 2 × 39 = 78
Therefore, the number of chromosomes in each of the dog's body cells is 78.
The answer is A. 78.