Answer: haploid + haploid = diploid
Fertilization is a process in sexual reproduction in which the male gamete fuses with the female gamete in order to produce the zygote which is the precursor of new organism. The ploidy of the male gamete cell is haploid (n) also that of the female gamete cell is haploid (n), when these two gamete cells fuse together they form a diploid zygote (n) cell. Hence, haploid + haploid = diploid is the correct equation.