Final answer:
Chromosomes first become visible during the leptotene phase, and the synaptonemal complex, which ensures the tight pairing of homologous chromosomes, is complete during the pachytene phase of prophase I in meiosis.
Step-by-step explanation:
Chromosomes first become visible during leptotene phase, which is the initial stage of prophase I in meiosis. During this stage, homologous chromosomes start to condense and become visible under a microscope, although they are not yet fully condensed. The synaptonemal complex is completed in the pachytene phase of prophase I. It is during the pachytene stage that the synaptonemal complex, a lattice of proteins between the homologous chromosomes, is fully formed. This complex facilitates the tight pairing of homologous chromosomes in a process called synapsis, which is critical for the exchange of genetic material between them through crossing over. Hence, the correct answer to the question "Chromosomes are first visible in ________ and the synaptonemal complex is complete in _______?" is d) Leptotene, pachytene.