The centriole undergoes duplication during:

The correct answer is S‑phase.

The cell cycle consists of several distinct phases that prepare a cell for division:

1. G1 phase: cell grows and synthesizes proteins needed for DNA replication.
2. S phase: the cell replicates its DNA. During this phase, structures that must be duplicated for the next division, such as centrioles, are also copied.
3. G2 phase: the cell continues to grow and checks that DNA replication was successful.
4. M phase (mitosis): chromosomes separate and the cell divides.

Centrioles are cylindrical organelles that form the core of the centrosome, which organizes the microtubule network during mitosis. For each daughter cell to inherit a functional centrosome, each original centriole must give rise to a new daughter centriole.

Because centriole duplication must be coordinated with DNA replication to ensure that the two daughter cells receive a complete set of organelles, the duplication event is tightly linked to the S‑phase of the cell cycle. The process begins shortly after the onset of DNA synthesis and is completed before the cell enters G2, so that by the time mitosis begins each cell contains a pair of mature centrioles ready to nucleate the mitotic spindle.

Therefore, the centriole undergoes duplication during the S‑phase of the cell cycle.