During mitosis, the primary focus is on the replication and segregation of cells. More specifically, the process involves the division of the nucleus (where the genetic material is) and results in the formation of two identical daughter cells from one parent cell.
However, if you're looking at what specifically within the cell is replicated leading up to and during mitosis, it would also include the replication of organelles and proteins, but these processes mostly occur during the preceding interphase, not during mitosis itself.
In terms of what replicates during the actual process of mitosis, it's predominantly the cells.
So, the correct response is: cells.