If one defines life as the simplest form capable of displaying the most essential attributes of a living thing, one very quickly realizes that the only real criterion for life is the ability to replicate. Only systems that contain nucleic acids are capable of this phenomenon. This sort of reasoning has led to a new definition of organism:

An organism is the unit element of a continuous lineage with an individual evolutionary history.

Consider the new definition of life. If you were to argue that a virus or a single mitochondria were, in fact, alive based on this definition, you would be ignoring the main tenet of
A) cell theory.
B) endosymbiosis.
C) nucleosynthesis.
D) natural selection.

The viruses which are considered the non-living organisms as they are not composed of cells and mitochondria which are evolved from the endo-symbiotic relationship with cells are also considered not considered a cell.

According to the question, these cells will have life and will be living which completely ignores the most important theory of biology called cell theory given by Schwann and Schleiden. This theory states that cell is the structural and functional unit of life and all living organisms whether unicellular or multi-cellular are made up of cell.

Thus, option A is the correct answer.