Which piece of scientific knowledge best supports the idea of the cell theory that cells come from existing cells? Cells are the smallest structures that carry out specialized functions in a living organism.
Chromosomes are copied during the replication process.
Multicellular organisms have chromosomes located in membrane-bound nuclei.
Some members of a species display a trait due to mutations in genetic coding.

Chromosomes are copied during the replication process.

Explanation:

According to the part of the cell theory proposed in 1838, cells come from already pre-existing cells. This means that a new cell arises from an old cell. The way by which this occurrs is between CELL DIVISION.

In cell division, like Mitosis and meiosis, new cells result as a result of the division of nucleus and cytoplasm of the cell. Therefore, Chromosomes are copied during the replication process best describes the idea of cell theory that cells arise from already existing cells.