In gymnosperms,the pollen chamber is cavity in the ovule in which pollen grains are stored after pollination. Within the pollen chamber, the generative cell nucleus divides to form two gametic nuclei and the tube cell elongates to form the pollen tube. The pollen tube penetrates the female gametophyte and the gametic nuclei (also known as sperm nuclei) pass through the tube. One of them unites with an oosphere (the female gamete) and forms the zygote (2n). Hence, option B is correct.