**Core Concept**
The commonest cause of carcinoma endometrium is related to the hormonal influence on the endometrium. The endometrium is a layer of tissue that lines the uterus, and its growth and maintenance are primarily regulated by estrogen and progesterone hormones. An imbalance of these hormones, particularly an excess of estrogen, can lead to unopposed proliferation of the endometrial cells, increasing the risk of endometrial carcinoma.

**Why the Correct Answer is Right**
The correct answer is **Unopposed estrogen**. Unopposed estrogen refers to the presence of high levels of estrogen in the body without sufficient progesterone to balance it out. This hormonal imbalance can lead to the uncontrolled growth of the endometrial cells, increasing the risk of endometrial carcinoma. This is often seen in postmenopausal women who are on hormone replacement therapy (HRT) without progesterone or have conditions such as polycystic ovary syndrome (PCOS) that lead to estrogen excess.

**Why Each Wrong Option is Incorrect**
**Option A:** **Obesity** is a risk factor for endometrial carcinoma, but it is not the commonest cause. Obesity can lead to insulin resistance, which can stimulate the production of insulin-like growth factor-1 (IGF-1), promoting the growth of endometrial cells.
**Option B:** **Tamoxifen** is a selective estrogen receptor modulator (SERM) used in the treatment of breast cancer. While it can have a protective effect on the breast tissue, it can actually increase the risk of endometrial carcinoma due to its estrogen agonist effects on the endometrium.
**Option C:** **Human Papillomavirus (HPV)** is a common cause of cervical cancer, but it is not the commonest cause of endometrial carcinoma. HPV can cause abnormal cell changes in the cervix, but it is not typically associated with endometrial cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the risk of endometrial carcinoma increases with age, particularly after the age of 50. Women with a history of unopposed estrogen exposure, such as those on HRT without progesterone, are at increased risk.

**Correct Answer:** D. Unopposed estrogen.