Final answer:
One sperm in double fertilization fuses with the egg to form a zygote, and the other sperm fuses with the polar nuclei to form endosperm, making option b. egg; polar nuclei the correct answer.
Step-by-step explanation:
In the process of double fertilization, one sperm fuses with the egg, forming the diploid zygote, and the other sperm fuses with the polar nuclei. Therefore, the correct answer is option b. egg; polar nuclei.
Double fertilization is a unique event in angiosperms wherein two separate fertilization events occur simultaneously. The first sperm cell fuses with the egg to give rise to the zygote, which eventually develops into an embryo. The second sperm cell fuses with the polar nuclei within the central cell of the ovule, resulting in the formation of the triploid endosperm. This endosperm serves as a nutritive tissue for the developing embryo, supplying the necessary food reserves during development. Unlike the fertilization process in mammals, where only the egg and sperm fuse, plants have this additional step to ensure proper nourishment and support for the embryo's growth.