Answer : The correct answer for the blank is-
reproductive cells.
Gametes are the haploid, sex cell (also called reproductive cells) that are produced in sexually reproducing organisms through the process of cell division, called meiosis.
During this process, a diploid mother cell produces four haploid gametes (such as egg and sperm) through two rounds of cell division (meiosis I and meiosis II).
The gametes produced by each parent fuse together to form diploid zygote (fertilization), which develops into fetus (through proliferation and differentiation).
Thus, reproductive cell is the right answer for the blank.