Hint: Pregnancy which is also known as gestation is the development of an offspring inside a female womb, which lasts for 9 months after which parturition takes place which leads to the birth of the offspring.
Several processes take place in between fertilization and birth of the offspring. When the semen containing sperm is released into the vagina, the sperm swims to the ampulla of the fallopian tube where it fuses with the ovum, this entire process is fertilization.
The fertilization results in the formation of a zygote and when this zygote moves to the isthmus of the fallopian tube the mitotic division takes place which is known as cleavage. Cleavage leads to the formation of blastomeres and when the blastomeres reach 8-16 in numbers then the embryo is known as a morula.
The morula continues to divide again and again to form a structure called a blastocyst. Blastocyst has two layers of blastomere cells, the outer layer known as trophoblast, and the inner layer known as inner cell mass. The blastocyst gets fixed into the endometrium of the uterus and it is called implantation.
Option D: After the blastocyst gets fixed into the endometrium the inner mass cell starts differentiating to form the embryo which leads to the beginning of pregnancy, Therefore, this is the correct option.
All other options are irrelevant.
Hence, the correct answer is option (D)
Note: Pregnancy is divided into three trimesters each of which is of 3 months, After the first trimester organs are formed in the fetus and after second trimester hairs develop on the body of fetus and after all three trimester the fetus gets fully developed.