What is the field of study that encompasses the complete set of DNA within a single cell of an organism?

The correct answer is genomics because this field specifically focuses on the comprehensive analysis of the entire set of DNA, known as the genome, within a single cell of an organism. Genomics involves studying the structure, function, evolution, and mapping of genomes, including all the genes as well as the non-coding sequences.

Genetics, while related, primarily deals with the individual genes and their roles in inheritance and variation, without necessarily considering the whole genome. Molecular biology focuses on the molecular mechanisms of biological activity, particularly at the molecular level involving RNA, DNA, proteins, and how these molecules interact. Cell biology, on the other hand, studies the structure and function of cells, but does not specifically deal with the complete set of DNA within those cells. Thus, genomics is the most appropriate term when discussing the entire DNA content in a cell.