What structure in a cell condenses to form chromosomes?

The correct choice is chromatin strands because chromosomes are formed from the condensation of chromatin during cell division. Chromatin is a complex of DNA and proteins found in the nucleus, and when a cell prepares to divide, it undergoes a process of condensation where the chromatin coils and folds into distinct, tightly packed structures known as chromosomes. This condensation is crucial because it allows the long DNA molecules to be separated and distributed efficiently to daughter cells during mitosis or meiosis.

The other options do not relate to the formation of chromosomes. The plasma membrane is a barrier that surrounds the cell, the cell wall (found in plant cells) provides structural support but does not play a role in chromosome formation, and the nucleolus is involved in ribosome production, not in the condensation of chromatin into chromosomes.