Final answer:
Diploid cells become haploid during meiosis.
Step-by-step explanation:
The process in which diploid cells become haploid is meiosis. Meiosis is a type of cell division that occurs in sexually reproducing organisms, and it involves two rounds of nuclear division.
During meiosis, each chromosome is duplicated during interphase, and then homologous chromosomes separate in meiosis I, followed by the separation of sister chromatids in meiosis II.
The result is the production of four haploid cells, each with half the number of chromosomes as the parent cell.