Implantation occurs at what stage of zygote?

Ans. is 'b' i.e., Blastocyst * The morula after spending about 3 days in the tube enters the uterine cavity via the narrow ostium (1 mm) on the fourth day in the 16-64 cell stage. Blastocyst formation is on the 5th day.* Implantation occurs in the endometrium on the anterior or posterior wall of the body near the fundus on the sixth day following fertilization (corresponding to the 20th day of the menstrual cycle).* The deeper penetration of the human blastocyst is called interstitial implantation, which happens by approximately the 13th day after fertilization.