**Core Concept**
The World Health Organization (WHO) technical series published guidelines for a Community Periodontal Index of Treatment Needs (CPITN) probe, which is a standardized instrument used for assessing periodontal health in populations. The CPITN probe is an essential tool for epidemiological studies and clinical practice in the field of periodontology.

**Why the Correct Answer is Right**
The CPITN probe is a small, standardized periodontal probe with a rounded end, used to measure the depth of periodontal pockets in teeth. The probe has a graduated scale, allowing for accurate measurement of pocket depths, which is crucial for assessing periodontal health and treatment needs. The correct answer is associated with the WHO technical series, which emphasizes the importance of standardized periodontal probing in clinical and epidemiological settings.

**Why Each Wrong Option is Incorrect**
**Option A:** The Community Periodontal Index of Treatment Needs (CPITN) probe is not also known as a "periodontal explorer." While a periodontal explorer is a type of instrument used in periodontal examination, it is not the same as the CPITN probe.

**Option B:** The CPITN probe is not also known as a "WHO probe." Although the WHO is associated with the technical series, the correct term for the instrument is the CPITN probe.

**Option C:** The CPITN probe is not also known as a "periodontal probe with a marked end." While this description is accurate, it is not the commonly recognized term for the instrument.

**Clinical Pearl / High-Yield Fact**
The CPITN probe is a crucial tool for assessing periodontal health in populations, and its standardized measurement is essential for epidemiological studies and clinical practice.

**Correct Answer:** C. Periodontal probe.