Final answer:
Chromosomes are first visible during prophase I, and the synaptonemal complex is complete early in prophase I, aiding in the process of crossing over.
Step-by-step explanation:
Chromosomes first become visible during prophase I of meiosis, and the synaptonemal complex is complete early in prophase I. During prophase I, chromosomes condense and become visible inside the nucleus. As prophase I progresses, the synaptonemal complex, which tightly binds the homologous chromosomes together, fully forms and supports the process of crossing over. At the end of prophase I, after the synaptonemal complex begins to break down, the homologous chromosomes, also known as tetrads at this stage, are held together only at the chiasmata.