Double fertilization occurs when
a. two sperm enter the ovule; one fertilizes the egg, one fuses with 2 polar nuclei
b. one sperm enters the ovule and fertilizes 2 eggs
c. two sperm enter the ovule; and fertilizes1 egg
d. two sperm enter the ovule; one fertilizes the egg, one fuses with the endosperm

a. two sperm enter the ovule; one fertilizes the egg, one fuses with 2 polar nuclei

Explanation:

Double fertilization is the characteristic feature of the angiospermic plants. After pollination, pollen grains deposited on the stigma of the pistil or carpel. The pollen grain on the stigma germinates forming a pollen tube. The pollen tube carrying two male gametes grows down through the style of the pistil and enters into the embryosac formed within the ovule and releases two male gametes.

One of the gametes fuses or fertilizes with the egg forming zygote. The other male gamete fuses with the two polar nuclei of the embryo sac. The fertilization between two polar nuclei and one male gamete is called the triple fusion. The triple fusion results in triploid primary endosperm cell (3n).

Thus, in angiosperms, there are two fertilizations called double fertilization resulting in the zygote (2n) and primary endosperm cell (3n).  

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