To solve this problem, we need to understand the changes in chromosome number during the cell cycle, specifically during the S phase of interphase in mitosis.$\newline \bullet$The fruit fly has 8 chromosomes in each cell, which is the diploid number (2n = 8).$\newline \bullet$During the $G_1$ phase, the cell has 8 chromosomes.$\newline \bullet$In the S phase, DNA replication occurs, but the number of chromosomes remains the same.