Cells go through a series of events known as the cell cycle as they grow and divide. During the cell cycle. a cell grows, prepares for division, and then divides to form two daughter cells. Each daughter cell then moves into a new cycle of activity, growth, and division. The cell cycle includes four phases — G1, S, G2 and M. What occurs during the S phase?

a) Cells increase in size and synthesize new proteins and organelles
b) There is intense growth and activity
c) Prepares the cell for mitosis
d) New DNA is synthesized as the chromosomes are replicated