The correct answer is b. the S phase.
During the cell cycle, the S phase, or synthesis phase, is when the DNA is replicated. This is a crucial step in cell division, ensuring that when the cell divides, each daughter cell will have an identical set of chromosomes. Prior to the S phase, during the G1 (Gap 1) phase, the cell grows and prepares for DNA synthesis. After the S phase, the cell enters the G2 (Gap 2) phase, where it continues to grow and prepares for mitosis. Therefore, option b is indeed the right choice as it specifically indicates the phase when genetic material duplication occurs.