**Explanation:**

The correct answer is option D, estrogen.

**Endometrium:**
The endometrium is the inner lining of the uterus. It undergoes cyclic changes during the menstrual cycle in response to hormonal fluctuations. The endometrium prepares for the implantation of a fertilized egg and provides a nourishing environment for the developing embryo.

**Hormones and Endometrial Proliferation:**
The menstrual cycle is regulated by various hormones, including estrogen and progesterone. These hormones are released from the ovaries in a coordinated manner.

**Estrogen:**
Estrogen is primarily responsible for the proliferative phase of the endometrium. It promotes the growth and thickening of the endometrium in preparation for pregnancy. Estrogen stimulates the proliferation of endometrial cells and increases the blood supply to the uterus.

**Ovulation and Estrogen:**
During the menstrual cycle, estrogen levels rise leading up to ovulation. Estrogen is produced by the developing ovarian follicles. As the follicles mature, they release increasing amounts of estrogen. This surge in estrogen stimulates the growth and proliferation of the endometrium.

**Endometrial Preparation for Implantation:**
Under the influence of estrogen, the endometrium becomes thicker and more vascularized. It develops a rich blood supply and increases the number of glands. This prepares the endometrium for the potential implantation of a fertilized egg.

**Role of LH and Oxytocin:**
While LH (luteinizing hormone) and oxytocin play important roles in the menstrual cycle and pregnancy, they are not directly responsible for the proliferation of the endometrium.

- LH triggers ovulation and the formation of the corpus luteum, which produces progesterone.
- Oxytocin is involved in the contraction of the uterus during labor and milk ejection during breastfeeding.

**Conclusion:**
In conclusion, the correct answer is option D, estrogen. Estrogen promotes the proliferation of the endometrium, preparing it for potential implantation. LH and oxytocin have other important roles in the menstrual cycle and pregnancy, but they do not directly stimulate endometrial proliferation.