**Core Concept**
Pelvic masses in women can be due to various causes, including ovarian tumors, uterine fibroids, endometriosis, and other gynecological or non-gynecological malignancies. A thorough evaluation involves a combination of clinical history, physical examination, and imaging studies to determine the nature and extent of the lesion.

**Why the Correct Answer is Right**
In this scenario, the primary concern is to rule out malignancy. Imaging studies such as ultrasound, CT, or MRI are essential to assess the size, location, and characteristics of the mass, as well as its relationship to surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because imaging studies like ultrasound are essential for characterizing the mass, especially when it comes to ovarian tumors, which are a common cause of pelvic masses in women.

**Option B:** This option is incorrect because a complete blood count (CBC) may be useful in assessing for signs of malignancy or infection, but it is not the primary investigation for characterizing a pelvic mass.

**Option C:** This option is incorrect because a pelvic examination is crucial in evaluating the mass, but it is not the only investigation needed. Imaging studies provide more detailed information about the mass and its relationship to surrounding structures.

**Clinical Pearl / High-Yield Fact**
A thorough pelvic examination, including bimanual and rectovaginal examination, is essential in evaluating the size, consistency, and mobility of the mass, as well as assessing for any associated symptoms such as pain or bleeding.

**Correct Answer:** D.