What kind of nuclear division must occur prior to sexual reproduction to ensure the offspring are 2n?

Meiosis must occur prior to sexual reproduction. Meiosis makes four haploid (n) cells, so when fertilization occurs, two haploid gametes come together and become a diploid (2n) cell. If meiosis had not occured, or if only mitosis occured, the the cell that was produced during fertilization would be 4n.