The correct option is A attachment of blastocyst to the uterine wall
The fusion of male and female gametes results in the formation of zygote which undergoes mitotic divisions called cleavage to form daughter cells called blastomeres.
The embryo with 8-16 blastomeres is called morula.
Morula continues to divide and transforms into blastocyst.
The blastocyst gets attached to the endometrium. After attachment, the uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst gets embedded in the endometrium of the uterus. This process is called implantation.
Egg released from the ovary moves by the movement of cilia in the fallopian tube. And if simultaneously the sperm also reaches the fallopian tube, it leads to fusion of the two gametes. Fusion of male and female gametes is called fertilisation.