**Core Concept**
The investigation of choice for endometrial sampling involves obtaining a tissue sample from the endometrium for histopathological examination to diagnose or rule out endometrial cancer, hyperplasia, or other abnormalities. This is crucial for early detection and management of endometrial pathology.

**Why the Correct Answer is Right**
The correct answer is **Endometrial biopsy**. This procedure involves inserting a catheter or a device through the cervix into the uterus to collect a sample of endometrial tissue. The sample is then sent for histopathological examination to assess for any abnormalities. This is the preferred method for endometrial sampling as it is minimally invasive, quick, and provides a high-quality sample for diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified. Since it was left blank, we will move on to the other options.
* **Option B:** This option is not relevant to the investigation of choice for endometrial sampling. Ultrasound may be used to assess the thickness of the endometrium or to guide the biopsy, but it is not the investigation of choice.
* **Option C:** This option is also not relevant. Hysteroscopy may be used to visualize the inside of the uterus and to perform a biopsy, but it is not the first-line investigation of choice for endometrial sampling.

**Clinical Pearl / High-Yield Fact**
It is essential to note that endometrial sampling should be performed in postmenopausal women or in women with abnormal uterine bleeding, especially if they have risk factors for endometrial cancer.

**Correct Answer:** .