DNA and cell division
The function of cell division is to increase the number of cells in an organism, however, for cell division to occur, a series of steps are required so that the genetic material remains the same.
Thus, before cell division actually occurs, there is a period in which the genetic material of that cell is duplicated so that this material is divided among the cells that will be formed.
So what happened in the nucleus to change the mass of DNA is that the DNA was duplicated. So alternative "A" is the answer.