9. During meiosis, the nucleoprotein complex that holds non-sister chromatids of a homologous pair together during crossing over is the _________________.

a. euchromatin
b. heterochromatin
c. synaptonemal complex
d. zygotene complex
e. spermatogonia

c. synaptonemal complex

To answer this question, we need to understand the process of meiosis and the structures involved. Meiosis is a type of cell division that occurs in sexually reproducing organisms, resulting in the formation of reproductive cells (gametes) with half the number of chromosomes compared to the original cell.

During meiosis, homologous chromosomes pair up and exchange genetic material in a process called crossing over. This exchange of genetic material promotes genetic diversity. The structure that holds non-sister chromatids of a homologous pair together during crossing over is called the synaptonemal complex.

Therefore, the correct answer is c. synaptonemal complex.