Give some stream of consciousness and then the answer. In humans, implantation is the stage of pregnancy at which the already fertilized egg adheres to the wall of the uterus. At this stage of prenatal development, the conceptus is a blastocyst. It is by this adhesion that the fetus receives oxygen and nutrients from the mother to be able to grow.

Answer this question: where does the implantation of a fertilized egg occur?
To answer the above question, the relevant sentence is: In humans, implantation is the stage of pregnancy at which the already fertilized egg adheres to the wall of the uterus.
Therefore, the answer is the wall of the uterus.