The answer is haploid + haploid = diploid.
Fertilization is the process in which two gametes (one male and one female) fuse which first
creates a zygote and then leads to the development of an embryo. Germ cells are cells
that give rise to the gametes - either egg cells (female gamete) or sperm cells (male gamete). Germ cells are
the only cells that can undergo both mitosis and meiosis. They are diploid
cells present in gonads. After meiosis haploid gametes are produced. Thus, in fertilization two haploid gametes fuse (n+n) and form diploid zygote (2n).
