Final answer:
The pollen tube releases its sperm and tube nucleus into a synergid, which is part of the female gametophyte involved in angiosperm fertilization. This event is a component of double fertilization, leading to the development of a zygote and endosperm.
Step-by-step explanation:
The process being described pertains to the fertilization event in angiosperms, which is known as double fertilization. When pollen arrives at the stigma, it germinates and a pollen tube grows through the style and into the ovule, carrying the sperm cells towards the female gametophyte. The pollen tube releases its content into a synergid, which is part of the embryo sac located near the egg cell. Upon reaching the female gametophyte, one sperm cell fertilizes the egg, forming a diploid zygote, while the other sperm fuses with the two polar nuclei contained within the central cell of the embryo sac, leading to the formation of the triploid cell that becomes the endosperm. Therefore, the correct answer to the question is that the pollen tube releases its sperm and tube nucleus into:
c. a synergid.