**Uterus as the Site of Implantation**

The uterus plays a crucial role in the process of implantation, which is the attachment of the blastocyst (early-stage embryo) to the uterine wall. Successful implantation is essential for the establishment of pregnancy. The uterine lining undergoes various changes during the menstrual cycle in preparation for implantation.

**Layers of the Uterine Wall**

The uterine wall consists of three layers: the endometrium, the myometrium, and the perimetrium. The innermost layer, the endometrium, is the site of implantation. It undergoes cyclic changes in response to hormonal fluctuations, allowing for the implantation process to occur.

**Preventing Implantation at an Abnormal Site**

To ensure proper implantation, it is crucial that the blastocyst attaches to the endometrium and not to any other location within the uterus. There is a layer that prevents implantation at abnormal sites. Let's discuss the possible options:

**a) Zona Pellucida:**
The zona pellucida is a glycoprotein-rich layer that surrounds the fertilized egg (zygote) and the early-stage embryo. While it plays a vital role in fertilization and early embryonic development, it is not involved in preventing implantation at an abnormal site. Therefore, the zona pellucida is not the correct answer.

**b) Trophoblast:**
The trophoblast is a layer of cells that forms the outer layer of the blastocyst. It is responsible for implantation and subsequent placenta formation. The trophoblast invades the endometrium and establishes a connection with the maternal blood supply. However, it does not prevent implantation at abnormal sites. Therefore, the trophoblast is not the correct answer.

**c) Corona Radiata:**
The corona radiata is not directly involved in implantation. It refers to the layer of granulosa cells that surrounds the oocyte within the ovarian follicle. It functions to protect and nourish the oocyte during its development. Therefore, the corona radiata is not the correct answer.

In conclusion, the correct answer to the question is none of the options provided. The layer that prevents implantation at an abnormal site is primarily the endometrium itself. The endometrium undergoes cyclic changes, including the development of a receptive state during the mid-secretory phase of the menstrual cycle, which allows for successful implantation of the blastocyst.

A) zona pellucida