The structure of chromosome can be observed best during

The structure of chromosomes can be observed best during Metaphase.
Metaphase is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage. These chromosomes, carrying genetic information, align in the equator of the cell before being separated into each of the two daughter cells.
During metaphase, the nucleus dissolves and the cell's chromosomes condense and move together, aligning in the center of the dividing cell.