After fertilization, the cells of the zygote divide many times producing a solid ball of cells called a a. gastrula b. yolk sac c. blastocyst d. morula

The correct answer will be option D (morula).

Explanation:

After fertilization or fusion of egg and sperm cells, zygote starts first cleavage division after 24 hours and form a solid ball of 10-30 cells called "morula" which can be seen after 96 hours or 4-days.

Morula cells do not show much growth as they are formed inside pellucid zone which cannot expand. This is the final stage formed prior to the formation of a fluid filled blastocoel cavity which further grows to form early blastocyst. The blastomeres on the surface of the morula fom extra-embryonic parts of the embryo while inside the morula develop into the embryo.

Thus, option D (morula) is the correct answer.