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.$\newline \bullet$Each chromosome is replicated to form two sister chromatids, but they are still considered one chromosome until they are separated during mitosis.$\newline \bullet$Therefore, after the S phase, the number of chromosomes remains 8, but each chromosome consists of two sister chromatids.$\newline$Thus, the number of chromosomes after the S phase is 8.$\newline$This corresponds to Option 1.