Question

what is exchanged during crossing over?

entire chromosomes
homologous parts of chromosomes
entire genes
entire chromosomes

Explanation:

Crossing over occurs during meiosis. It involves the exchange of genetic material. Entire chromosomes are not exchanged as that would be a different process (like non - disjunction in some cases). Entire genes are also not the unit of exchange. The process specifically involves the exchange of homologous (corresponding) parts of chromosomes. This allows for genetic recombination.

Answer:

homologous parts of chromosomes