Final answer:
Chromosomes first become visible during the prophase of the cell cycle, as this is when chromatin condenses into visible X-shaped sister chromatids.
Step-by-step explanation:
Chromosomes first become visible during the prophase stage of mitosis. While the G1 phase is a period of cellular activity involving the accumulation of DNA building blocks and energy reserves, the chromosomes are not yet condensed and therefore not visible under a light microscope. During prophase, however, the loosely packed chromatin coils and condenses into visible chromosomes, each with its identical partner attached, forming the familiar X-shape of sister chromatids. This condensation process renders the chromosomes visible to observational tools such as light microscopy, differentiating this phase from previous stages in the cell cycle where chromosomes are present but not visually distinct.