Hint: The development of embryo also called embryogenesis involved various steps starting from a single cell to a fully developed embryo. Cells of Rauber are formed during one of these steps of the embryogenesis process.
The blastomeres are arranged into an outer layer called trophoblast and an inner group of cells called the inner cell mass. Inner cell mass is like a small knob at one pole and is called the embryonal knob. This embryonal knob forms the embryo during embryogenesis.
Let us analyze the options to find out the correct option.
Option A: The blastocyst stage of the embryo has three parts called trophoblast, the inner cells mass and the blastocoel. Of this, the inner cell mass forms the embryo while the blastocoel is a fluid filled cavity. Therefore, this is the incorrect option.
Option B: Secretory cells of endometrium show effects of both progesterone and estrogen influence. They have other hormonal roles in terms of endometrium which is formed during the pregnancy and are not related to cells of Rauber. Therefore, this is the incorrect option.
Option C: Trophoblast cells in contact with the uterine wall do not form the cells of Rauber. Therefore, this is the incorrect option.
Option D: The trophoblast layer of the blastocyst is not involved in the embryo formation. It forms the external parts of embryos like extraembryonic membranes like chorion and amnion which provide protection and nourishment to the embryo. The trophoblast cells which are in contact with the embryonal knob are called cells of Rauber. Therefore, this is the correct option.
Hence the correct answer is option (D) Trophoblast cells in contact with inner cell mass of blastocyst.
Note: The cells of Rauber form a thin layer of trophoblast and overlies the inner cell mass layer of the blastocyst. The trophoblast cells in contact with the embryonal knob are known as cells of Rauber.