During prophase, a stage in the cell cycle,

a
the nuclear envelope breaks down and chromatin condenses into chromosomes.
b
the cell duplicates its genetic material- doubling the number of chromossomes.
c
the chromosomes in the cell begin to move and align at the equator of the cell.
d
the cell begins to separate into two new genetically unique cells.

a) the nuclear envelope breaks down and chromatin condenses into chromosomes.