Final answer:
In embryo sac development, the egg apparatus contains the egg cell and the synergids, which assist in guiding the pollen tube for fertilization.
Step-by-step explanation:
In the development of the embryo sac, which is the female gametophyte in angiosperms, the egg cell is part of the egg apparatus along with the synergids. The egg apparatus, which is located at the micropylar end of the embryo sac, contains one egg cell and two synergids. The synergids play an important role in guiding the pollen tube for successful fertilization, after which they disintegrate. Hence, the answer to the student's question is b. synergids. This differs from the central cell of the embryo sac, which contains the two polar nuclei and eventually combines with a sperm to form the triploid endosperm necessary for seed development. It is also distinct from the antipodals, which are three nuclei that position themselves opposite the micropyle and typically degenerate without direct involvement in fertilization.