The correct answer is c) Interphase.
Nucleoli are cellular structures found within the nucleus, and they are primarily associated with the production of ribosomal RNA (rRNA) and the formation of ribosomes. During interphase, the cell is actively growing and preparing for division. This is the phase when the nucleoli are most prominent and functional, as the cell synthesizes the necessary components for protein synthesis.
In contrast, during prophase, anaphase, and prometaphase, the nucleolus typically becomes less visible or may even disappear as the cell transitions into mitosis. Therefore, the nucleoli are best observed and are most active during interphase, making it the correct answer.