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.
The correct answer is d. morula. <span> A morula is a stage in early development, formed 3–4 days after fertilization. It is a 16-cell mass which forms a solid ball contained within the zona pellucida. A morula develops into a blastocyst.</span>
