In which phase does organelle duplication predominantly take place?

Organelle duplication predominantly takes place in the G₂ phase of the cell cycle. During this phase, the cell undergoes a series of preparations for mitosis, ensuring that it has the necessary components and energy reserves for cell division. Key events include the final checks for DNA replication and the replication of organelles such as mitochondria and the endoplasmic reticulum, which are crucial for providing the daughter cells with the organelles needed for their metabolic activities.

The G₁ phase is primarily focused on cell growth and the synthesis of proteins required for DNA synthesis, while the S phase is specifically when DNA replication occurs, but organelles are not the main focus of this phase. The M phase is dedicated to mitosis and cytokinesis, where the cell divides, but it does not involve organelle duplication. Thus, the G₂ phase is the correct answer as it is the phase where the duplication of organelles is most significant to prepare the cell for division.