Final answer:
In angiosperms, double fertilization occurs when one sperm fertilizes the egg to form the diploid zygote, and the other sperm fuses with the polar nuclei to form the triploid endosperm.
Step-by-step explanation:
In angiosperms, double fertilization occurs when one sperm fertilizes the egg to form the diploid zygote, while the other sperm fuses with the polar nuclei to form the triploid endosperm. This process is essential for the development of the embryo and provides nourishment to support its growth.