Final answer:
Chromosomes are most compact during mitosis, especially at the metaphase stage. They are less condensed in interphase, including S phase where DNA replication occurs.
Step-by-step explanation:
Chromosomes are in their most compacted form during mitosis, specifically in the metaphase stage of the process. This is when chromosomes are aligned on the metaphase plate, tightly wound and easily distinguishable. Throughout interphase, including the S phase when DNA replicates, chromosomes are in a less condensed chromatin state. However, as the cell progresses into mitosis, starting with prophase and peaking at metaphase, the chromosomes become maximally condensed and can be clearly observed under a light microscope.