Chromosomes condense and become visible during prophase, the first stage of mitosis or meiosis.

Here's why:

* Condensation: During prophase, the long, thin strands of DNA (chromatin) coil and pack tightly into compact, visible structures called chromosomes. This makes it easier for the chromosomes to be separated during cell division.

* Visibility: The condensed chromosomes are now thick enough to be seen under a light microscope.