During mitosis, the primary focus is on replicating the genetic material, specifically the chromosomes, which consists of DNA. However, the options you've provided do not specifically mention chromosomes or DNA.
Among the options given, "cells" would be the best choice, as the process of mitosis results in the division of a single cell into two identical daughter cells. However, it's important to note that before mitosis begins, DNA and many organelles are replicated during the S phase of interphase.
So, the best response from your list would be:
cells.